i like locked diffs because it helps keep power to both sides, like when you go into a turn the inside wheel lifts off the ground a bit and spins faster
take a look at this video at .12 secs for example, because he has no locked diff the inside wheel spins and smokes but the outside wheen doesnt spin at all
http://www.youtube.com/watch?v=o5m8bXguQ-0
the same thing happens to me in xmods and unlucked rear diff cars normal go alot slower in the turns because of this
edit: this is the same suv with a locked diff just to show you the differance
http://www.youtube.com/watch?v=Qlv_F2GZ2Ds&NR=1