I would like this feature. But the problem is, you'd still be down one person, which might screw with people who are in queue. Say for instance.

Queue in as a DPS for Halatalii HM.

You're in the dungeon. Tank decides to dip. You then commence a role swap. The tank who was in queue for that spot might end up getting screwed due to the role swap happening. So you'd need to wait on a DPS to fill the slot in that you originally had when you first got into the instance.