Here's the scenario: You've rolled into a roulette. You don't know any of these people. This isn't some serious power-gamer environment where every living body counts. Where do you personally draw the line in your health-keeping duties? At what point does "I've failed as a healer" turn into "you deserved that, dude"?

For me personally it's a combination of two main factors. Vulnerability stacks is loosely one of them. I'd say 1-2 stacks is generally something I'd heal through. I start getting annoyed at 3, and 4 or more if they get slammed by something I'm less likely to consider the death my fault. The other more important factor is where it looks like those stacks are coming from. If it seems like the person is derping out or doesn't know mechanics, that's one thing. If they're clearly just standing in something repeatedly because they expect me to keep them up no matter what they do, that's where I get vindictive. At that point it goes beyond whether I consider it my job to keep them alive or not; if someone clearly expects to forego the bare minimum of effort in order to force me to carry the job of paying attention to this dungeon/trial/whatever, then I'll toss a pity heal out on occasion, but I'm not going to shed any tears if my next heal is a Raise.

Pull up a chair everyone. Let's bring out these selfless martyrs who'll heal through the dreaded 8-stack vulnerability in a guildhest. The wicked witch healers who cruelly kill their parties for fun. When do you let 'em die?