It is so, so unbelievably inconsiderate, especially since our group was struggling and really needed the extra DPS. I mean, you can't stay a few more minutes to help out? Really? We finished, but barely, and the healers were completely drained.
The person just "disconnected" and then hopped onto another character. E.g., the disconnected character "left" the group. I've seen this happen several times before, but I didn't realize why and thought it was strange. One epiphany later, and now I, sadly, realize why. Yes, I'm slow. But people who do this should be banned from the game.
PS: I have a screenshot and name this time.