I drop them all the time when I need like say 100k more exp, I'll do a dungeon until I get that much and then drop. Just kidding lol if only that easy. I rarely drop but have a few times before. Last time was a week ago (before that I can't remember) but had a summoner in the party and they would not stop using Physik on everyone (I'm a WHM). Everyone would be at 100% HP, and the tank would take one hit and go down a sliver, and sure enough Summoner casting Physik on him. Other DPS would take a small hit (the kind that most healers would actually ignore because they will regen it themselves anyway, plus I keep Medica II up as well). Was getting annoying because it wasn't like I couldn't heal, it was like around a level 40 dungeon where no one was really taking damage and no one ever wiped, so definitely wasn't because I was lacking, they just wouldn't stop. I finally told them to give it a rest because it was making the other pulls take foreverrrrrrr because we basically had 1 DPS and 2 healers. When I asked they DPS and not Physik since I have it under control, they literally just stood there spamming Physik on me over and over and over, then the tank told me to lighten up that they can do whatever they want! So I just stopped healing the Summoner only everyone else since I was like you seem to have it under control, in which case the summoner of course wiped super fast, but by that point we weren't even at the first boss yet and were pushing 25 minutes into it so I was like screw this, told them have fun finding another healer and dropped.

I'd never drop just because I was mad or the group was bad, but when people are purposely being a jerk or playing their role totally wrong, making everything take 3x longer than it should, or giving attitude non stop, then I have little patience.