LSD's have a lot of advantages beyond just making your corner exit faster. When I first learned about them I was told they help corner exit speed, and that's like kind of "meh", I don't care about my lap time, so it's hard to get too excited about that. But they do much more.
They make power oversteer much more predictable and controllable. Your two rear wheels are semi locked together so you know how fast they are spinning and thus how much grip you have back there. With an open diff, if you push the throttle, you might just lose grip in one wheel, then when you apply power you spin that wheel but still have grip on the other wheel. If you want to fool around with drifts or donuts or whatever, an LSD is much more fun. It also is safer in the sense that your car is predictable, it's less random whether you oversteer or not.
Without an LSD you can get a nasty chirping/hopping through tight turns. What happens is one wheel lifts, and you speed it up, then it connects to pavement again and you get a sudden jerk and chirp. Through a tight turn (especially with some elevation change) you can get several lift-fall events so you go jerk-chirping around the corner. Not cool.
There are two main types of diff used in sports cars. One is Quaife/Torsen mechanical type, the other is clutch-pack friction disk type. Some random things I have read about these diffs :
Clutch-type diffs can have variable amounts of "preload" and "lockup" or "dynamic" locking. Usually this is expressed as the % of lockup (eg. 0% = open diff, 100% = solid axle), but sometimes it's expressed as the amount of force through the clutch pack. "preload" is the amount of lockup with no force coming through the drive shaft. If you lift the wheels up and turn one side and see how much the other side turns - this is the "preload". Preload can make the car hard to make tight turns - it eliminates the ability to turn one wheel without turning the other, so street cars often have 0% - 20% preload. The housing of the LSD has this ramp built into it and when force comes through the drive shaft, it pushes a pin against the ramp which forces the clutch plates together, creating more lockup. This is the "dynamic number". You will see LSD's described as 20/40 or something, which means 20% preload and 40% under force. Sometimes this is also described as the "ramp angle" because the angle of that ramp determines how quickly the LSD adds more pressure.
Race cars use LSD's with 40/60 or 50/80 settings. This is lots of lockup. This works for races because you are never actually making super tight turns on race tracks. For Autocross people generally use less preload or a Torsen diff. If you have a high lockup, you can only take tight turns by drifting the rear end. Preload also greatly increases low speed understeer. Most OEM LSD's max out at 20-40% (Porsche Cayman/911 non-GT3 LSD maxes at 30%).
Torsen diffs act like zero-preload , and also don't provide much lockup under braking. Clutch type LSD provide stability under braking - they keep the rear end straight, because a wiggling rear end can only happen if the rear wheels are spinning at different speeds.
Clutch type LSD's wear out and have to be serviced like any other clutch plate. TBD's generally don't need much servicing unless they are beat up hard, roughly like transmission gears. Many OEM LSD's have very low settings so you would want to replace them anyway. It may still be wise to tick the OEM LSD option because it can be easier to fit an aftermarket LSD if you had the OEM one in the transmission (details depend on the car). OEM clutch-type LSD's also often wear very quickly; when buying a used car "with LSD" it is often actually an open diff because the LSD is shot.
BTW Lotus doesn't fit LSD's, and the new McLaren has no LSD. If your goal is sharp steering and fast lap times, then an LSD is not a clear win. If your goal is the "driving pleasure" of being in good control of your rear end slip angle, an LSD is more obviously good. See Lotus Evora with no LSD for example.
In non-slip scenarios, LSD's increase understeer by binding the sides of the car to each other. (this is why Lotus prefers not to fit them). In some cases, it may be that ticking the optional LSD box actually makes a car worse. For example this may be the case with the Cayman S, where the LSD added to the standard suspension increases understeer too much ; in contrast, the Cayman R has LSD standard and the suspension was set up for that, which involves more camber and stiffer sways, both of which are anti-understeer moves.
Some commentators have been knocking the MP4-12C for not having an LSD, but those commentators "don't get it". The MP4 is a computer-controlled car, like the Nissan GTR or the Ferrari 599. It does not rely on mechanical bits for power transfer, stability under braking, control of over/under-steer / etc. Traditional mechanical bits like LSD or sway bars or spring rates or whatever just don't apply. You don't need an LSD to keep you straight under braking when the car is doing single-wheel braking based on the difference in steering angle and actual yaw rate. Normal car dynamics is a balance between twitchy vs. safe, or turn-in vs. stability; Lotus and McLaren have bent the rules and avoided this trade-off, however that means less hoon-ability. See for example : Lewis Hamilton fails to do donuts in the MP4 .
It's much easier to kick a drift and countersteer through it correctly if you don't have to think through it rationally. It's almost impossible to catch the countersteer fast enough and with the right angle if you are doing it with your conscious mind going through the steps. What you have to do is get your body to do it for you subconsciously. Fortunately there's an easy way to do that :
It just requires using the old adage "look where you want to drive, and drive where you look".
Step by step :
Start steering into the corner.
Choose a spot out in the distance that is in the direction you want to go after the corner.
Stare at that target point and keep your eyes on it the whole time.
Kick your drift (power-on oversteer, clutch kick, whatever you want to do)
Point your steering at the target point - keep looking at the target point and just keep the wheel centered on it.
You will automatically transition from steering into the corner to countersteer as the tail comes around.
This helps you countersteer quicker, and also helps you to not over-do your countersteer (ala Corvette club), because you aren't thinking "countersteer", you're just thinking "point the wheel where I want to go". The standard mistake to catch it too late, and then over-do it because you are thinking too hard about "countersteer" so then you fish-tail in the opposite direction. You do not have to "countersteer", all you have to do is point the front wheels in the direction you want to go.
(* note : this is not actually true, the correct steering angle is slightly past pointing the wheels where you want to go, but I find if I think this way, then my hands automatically do the right thing).
It really is true that you should "look where you want to drive, and drive where you look" ; it's one of those things they tell you in race school and you're like "yeah yeah duh I know, let me out on the fucking track already", but it doesn't sink in for a while. 99% of the time that I make a mistake driving, I realize after the fact that I wasn't looking where I wanted the car to go. You want to be looking far ahead, not ahead on the ground right near you. When I really fuck up I realize after the fact that I was looking at my steering wheel or my shifter, eg. looking down inside the cabin.
One thing that does help is to put a piece of tape on the top of your steering wheel (if you don't have a mark built in). Again this seems totally unnecessary, you will think "I know where fucking straight ahead is, I don't need a mark", but it does help. What it does is give you a *visual* cue of where straight ahead is, so it connects the eyes to what the hands are doing. You want that connection of visual to hand motion to be subconscious, to avoid the rational mind. Your hands perform better when they are out of conscious control.
I got a lot of bad information when I was shopping for cars. You have to ignore all the mainstream press, Edmunds, Motortrend, all that kind of shit. The professional automative writers are 1. lazy (they don't actually research the cars they write about, or how cars work in general), 2. corrupt (they praise cars so that they can get free testers), and 3. just generally stupid. They don't write about what you actually want to know or what would actually be useful. Furthermore, the idea that you can tell a lot from a test drive is I believe not true. One thing I have to get over is I'm afraid to really thrash cars in the test drive, and without a really hard thrashing you can't tell how they behave at the limit. For example, you'd like to know things like what does the car do if you mash the brake while cornering near the limit of grip? The auto writers won't tell you and you can't find out in a test drive.
A good source of information is web forums. Not your normal "I love my car"/"check out my bling" web forums, but the actual racer's forums, like the SCCA and other groups (spec miata, spec boxster, etc, lots of racer forums out there). There you can read about the things that really matter in a sports car, how it acts on the edge, how much prep it needs to be track-worthy, whether the engines blow up on the track, etc. Even if you don't track and have no intention of tracking, the guys who do tend to be a better class of humans than the normal web forum car people, who care more about "brand history" and rims and who has a faster quarter mile.
The vast majority of modern sports cars are set up for understeer (eg. Porsches, BMW's, etc.). The only ones I know of that aren't set up for understeer are the RX8 and S2000. Of course most of the retarded mainstream press says absolutely nothing about the intentional understeer (which you can fix pretty easily) and they say that the RX8 and S2000 have bad handling because they are "twitchy" or "bad in the rain". That's retarded. Those cars have great handling in the sense that it is predictable - if you go around a corner fast in the rain and jab the brakes, you will likely spin. That's not bad handling, that's a bad driver.
To some extent, "sharpness" or "liveliness" inherently go together with "twitchiness" or "dangerousness". For example cars that give you a nice little bit of oversteer will generally also give you lift-off oversteer, which you probably don't want. The ideal thing would be a car that you could press a button to make it milder and safer for normal use, or twitchy and sharp for fun times. This is sort of impossible with traditional mechanical suspension, you can only get a good compromise. This is why the new crazy cars from Ferrari and McLaren use computers to control the handling. The Ferrari approach is a bit like the "Euro Fighter" - they make a car that is mechanically inherently unstable, and then use computers to keep it under control. The McLaren MP4-12C approach uses crazy new techniques that nobody else is using.