The rez is a really big deal in PVE imo.
When I first started boxing, I did 5 Warlocks. I thought, wow this is going to be so cool, 5 soulstone, 5 pets...Drain Life Healing...
We went to SFK and one toon died, soulstone!
Then he died again...
I then had to spend lots of time running him back into the instance.
After about an hour of this I switched out the mage for a priest and never regretted that decision.
In BC I never did Heroics with them, I don't remember how far I got, I was able to get the keys for Kara...Oh, I was stuck on BM, that last dragon.
My 4x Mages do worse than my Warlocks because there are no pets.