Well, I'd rather have a disc priest than a shadow priest at that level. They do more DPS (if they don't slack, obv.). Aside from that, yes, low level dungeons suck. But I don't even bother looking at DPS in them. The tank does the most DPS anyway, since most DPS specs don't have an AOE ability (and because vengeance).
It's unbelievable how susceptible this is to grieving. If people knew you could get banned from LFD for a week if you kick someone enough, they'd kick more often.
Not to mention the fact that low geared lvl 90 players often get kicked from HC dungeons because their DPS is low, and then Blizz would be like "hey, you got kicked twice, here's a week long ban".