Not only is it a waste of mana when there's a group wipe, its a tremendous waste of time.
Healer runs back. Party picks noses.
Healer drinks because it will take a full mana bar. 30 seconds.
Healer starts resses. Healer flustered by repeated REZ ME cries. 50 seconds.
Healer is out of mana, sits down to drink. Party picks noses. HEAL PLZ. 30 seconds.
Healer heals party. Healer is out of mana. Warlock lifetaps to 10 hp. 30 seconds.
At this point, the party probably starts to pull. The healer has no mana, and the Warlock is nearly dead.
While a nice narrative, it actually doesn't work that way in reality.
What actually happens is that the healer runs back and arrives, full mana (since he had plenty of time while running to regen it). He then rezzes everyone, drinks, and you go.
If you instead force everyone to run back, there's a near certainty that someone will be slower than the healer (who, after all, is probably the most experienced at 'running back to the dungeon' - by a couple of minutes - and everyone will have to wait for them.
The "I don't rez on wipes" is another one of those issues where people shoot themselves in the foot for the sake of some sort of vague 'fairness' principle. Someone
has to run back to rez. 5 people do not.