I really like unit fillers and have seen some great ones. My new local GW manager has made a 'bar' scene with an ogre barman and a bunch of drunker empire militia for one of his units - in fact all the best ones I have come accross seem to be for empire armies.
The general trend I have seen is for them to be for 6 men, and that seems to be quite acceptable everywhere.
The only ones I really use are zombies rising from the ground, and they are more spare markers for my undead army.
Probably the main reason we dont see many fillers in warriors of chaos is that for one you dont need all that many models to make a unit in the first place and for two the box sets tend to be a bit 'effecient' with their spare parts so I am not really left with enough bits to do much with other then have a mound of weapons and severed heads... but a decent pile of stuff with a few marauder guards would more definately look like a fine filler.