This happened during the last boss fight in Qarn (NM). No one was killing the pin things that spawn and hit the entire group after being left alive long enough. I didn't have time to sit on the DPS with cure since Regen alone wasn't enough to keep the tank alive and there was also the fact that there were more pins on the way that weren't being killed.
In such a case I think it's fine to prioritize keeping the tank alive since if they die they're going to be revived with low MP/TP (and that's assuming the rest of us don't die soon after somehow).
Oh definitely keep the tank up, that's priority #1. But Regen and Cure take the same time, Cure requires you to stand still but Regen puts you on GCD for 2.5secs anyway(or whatever it's down to with your spellspeed) so you can't do anything for the same amount of time, other than moving. If everyone was taking damage, then what you really wanted to cast is Medica though. It's not efficient to heal like 2people, but if 4people are damaged, then it's extremely mp efficient, and it's also more time efficient than trying to heal all 4.
The thing about casting regen on DPS is while it works well in early dungeons where damage tend to be non lethal even if you get hit several times in a row, in higher dungeons you generally die if you don't get a heal and get hit again, and Regen takes a while to actually top people off. Depending on the fight that's fine, but quite often just hitting them with a Cure is going to have a better result, both on your mana and on their survival rate. And if you really need a big heal, that's where Cure II comes into action with its fast cast and terrible mana cost, but that's for when you know that DPS is getting hit again soon and he's not healthy enough.
Now to go back to first point though, if they're being idiots and the tank is also low, let them die. Eventually you get more tools to deal with this situation though, like the cooldown that increases spellspeed a lot so you can quickly medica+cure II the tank and get everyone back up to full, or even Benediction when shit really hits the fan.