It's a useful tool indeed. And the best source of "control" for your experience. Which is why I feel the DF being such a random mishmash crapshoot should have much lower standards, I.E. Pass content is good enough.
Griefing being underperforming or doing something to antagonize on purpose, I think a lot less people actually do it than people seem to think. It seems a lot of people seem to take someone halfheartedly doing daily content as a personal affront.. when it isn't. it's just someone going through the motions. Again though it's much easier to just slog through someone's underperformance than to try and drag someone into a higher level, especially if they are a key role. I don't feel it's particularly fair from an objective point of view, but it's such a minor thing I also wonder why people take it so seriously.
Admitted that the healer could perform better, but is it really anyones place to push beyond their personal resistance after the initial "hey, I'm getting really close to dying every pull, can I give some healing rotation advice?"
And at that point it has to be also considered that it's not a personal affront and at very worst the parties involved can either remove themselves or the offending member. I don't like anyone AFK-ing in MSQ DF runs really.. but at the end of the day it's like.. 5 minutes longer. Yeah it's a cruddy thing to do, but is it worth getting worked up over?