While I do DPS, I can see why most healers would refuse to do it now, as most tanks are so undergeared and poorly using CDS or nto at all, and always die in a single GCD.
Pretty much every run, and i do mean EVERY run, is pretty much;
Tank pulls, goes down way too quick, have to consume oGCDs at the start, to "catch up", start with cast time reduction 10%, heals increased by 20%, ground regen, and normal regen on the tank, now that they are currently at full, start casting cure2, right as cure2 ends, and it triggers the heal, they are slightly above half HP, spam pressing cure2, waiting for the GCD to be ready. Casting cure2... and mid way or right as the cast finishes, tanks dead.
The GCD is just way too long to get any more "healing" off in time, much less DPS.
If DPS wasnt taking up the GCD, then this would be a different argument.
There just isnt time to cast DPS spells in most groups, so even if YOU are a decent tank, most tanks right now arent.
Heck, ive had some decent ones, with decent gear, and decent CD usage, die to just 3 mobs.
they are sitting at 90-99% hp for a while, I think, ok, what is one single stone spell going to hurt?
Right as the cast bar is about done, they are suddenly below half HP. If I used all my CDs the prior pull, which is usually the case when they tend to pull big, I can only cast cure2, and hope it finishes in time, nope. dead.
While this wasnt really an issue in 3.x series, this was a lot more common in 2.x series before they nerfed the amount of dmg tanks take in dungeons for 3.x (though there were undergeared tanks who tried big pulls in DPS stances, which was just as bad.)
So Im also reluctant to DPS, until I see my tank actually able to survive the ~2GCDs it takes to land a damage spell, and followed up by a cure spell.
(Swift cast is not much help either, as some tanks go down even faster than that.)
Maybe one tank out of 20+ dungeon runs might be ok enough to land some dmg occasionally, but even then, ive yet to have a tank in the 60+ range decent enough to do damage, w/o risk of a wipe.
TL;DR it only takes ONE GCD for a tank to go from full Hp to dead.
(doesnt help DPS like to take hits too, so if you even THINK of doing an AoE heal, or spending 1 GCD to heal them or yourself, you're risking the tank dieing. Also dont think of using oGCDs or else your stuck unable to save the tank, when spike dmg suddenly brings them to 1% hp mid GCD)
The ONLY reliable way to keep a tank up in 60+, is to PRECAST cures, even if they are at full HP.
The only reliable way to DPS, is when there are 2 or less mobs left.
(Id argue 3, but ive seen some bad tanks who wont CD for 3 mobs, or wont stun 1 of the mobs when able, leading to nearly being crit to death in a single GCD)