I rarely drive FWD cars (Except the UF1 which has open diff only) and I use LSD in all cars, mainly because my driving style relys on a lot of lift off oversteer and Locked diff does quite the opposite, so I am faster with LSD.
I support Dark Elite on this topic, LFS is a sim for me, and I try to make as close experince to real life as possible, hence why I use 720* rotation of wheel and H-shifter. It's not making me faster than I was using a cheap wheel (for now alteast), but it only makes much more proud when I leave racers who "don't use lfs" properly behind
One the side note, LSD is as fast and in some occasions even faster than locked diff, it just require a fine tunning...