I know what you mean, Zohar. It's seriously foolish and makes it much harder on the healers to keep you up in most circumstances. Worst offenders of this are the tanks that don't pop anything and proceed to try to speed run. Yeah...that's not going to end well for anybody there.
I suspect they do this because they think they can handle it along with the healer (why take this chance?), or they're fools that don't know how to play the class.