It would be nice to have this as an option when someone drops or leaves the group and a replacement needs to be found. I've had several when the tank has dropped and I've been playing dragoon (main is a paladin) and would have been happy to switch for the run.

I suspect the reason you can't is for the duty finder bonus. If switch in were possible it may be possible to abuse by queuing as a high level tank for the bonus, getting a low level dungeon the changing to a low level tank (or other class) and getting a massive xp reward.