Had this happen the other night in Thousand Maws. The healer (who looked to be in either CT or Allagan gear, I don't quite remember since I only saw him in the load-in cutscene) immediately disconnected as soon as we started. Clearly, only there for the quick tomes and didn't care about the group.

Joke's on him though, our summoner was awesome at keeping us alive, and we 3-manned the dungeon.