If used properly I think they could help. i.e. 2 rogues wind up with a 5% difference in damage, with same build and items, the one could tell the other how to up his dps. if they're just there for DPS, then they *should* be fairly close, but if they're not it could help him improve.
while DPS isn't the only thing, it is really important. I remember 3-manning VC (we were appropriate level) with a priest, druid, and rogue. I, the rogue, simply lacked the DPS of 3 people, and it made it very hard because the priest would go OOM. so while DPS may contribute to epeen, it also contributes to the group.