So we all do know that the slime spawn is currently very bugged, so until Notch releases his own fix, i would rather turn them off completely.
But thats not the only point:
In SMP there are 2 kind of slime spawns - server ones and bugged client ones. My guess is that the reason of such behavior is that SSP spawn is bugged so much that it doesn't turns itself off while in SMP. So by disabling both slime spawn and their "divide" capabilities we should be pretty much fixing client-side slime spawning in SMP.
P.S. Please do not post some useless crap like "just relog lol" and such - that is not an appropriate solution.