I would agree that it does very much depend on the car in question, for example I find that most FWD cars are better suited to a 1-way LSD.
I normally run a FC LSD on most set-ups and fail to see how this would be unrealistic as it allows you to set the locking points? Do you actually have any idea of the level of adjustment that can be reached with some top level motorsport diff systems? The diffs on a WRC car can be set to offer a different locking setting at different throttle possitions. Far more adjustability that can be done in GT4, I would argue that we can't do enough in GT4.
If you can pick up a copy of Richard Burns Rally, now that will give you an idea of the level of adjustability that we should have, not just with LSD's but tuning as a whole. The entire range of settings options in GT4 is just too limited in my opinion.
BTW - one to watch are the cars in reality that are fitted with very complex and advanced diff systems, such as the R33 & R34 Skylines, a lot of these cars actually suffer when you fit a LSD of any kind.
Regards
Scaff