I do that occasionally too whenever one of my FC mates has had a tank ragequit on them midway through a run and I'm trying to join their instance to help out. Unfortunately it's pretty standard for Tanks to have no queue waits, and a few of them will spam the join button trying to find one of those elusive "already 75% of the way through the dungeon" parties (in order to decrease time spent tome farming...)

Only thing I can suggest is that they put a "ONLY JOIN PARTIES IN PROGRESS" check option in.
That'd weed out most of the misfits.