In WoW instances, why does everyone think imps are better than voidwalkers for destro warlocks?

I'm a destro-specced lvl 80 lock who just recently started doing instances on anything resembling a regular basis. I've been told (chastised is more like it) that I should be using my imp instead of the voidwalker. I can't understand why, though. Imps are wimpy creatures that are killed very easily, and I don't see that their firebolts do much more dmg than the vw's attack. Plus vw's are good at holding threat, which is important because my fire spells generate threat up the wazoo. Finally, in a pinch I can use the vw's Sacrifice ability which buys me a few extra precious seconds to heal up or finish off a mob if my hp's get critical.

So I guess I'm just having trouble understanding why locks who use vw's in instances are treated like noobs. My dps isn't any different using an imp and the extra few hp granted to the group also seem kinda insignificant, especially if the tank & healer do their jobs.

Insight, anyone?