From experience in WoW, this would result in people queuing as Tank or Healer to get short queues, and then switching to their DPS job. You do not want this.
Quote Originally Posted by SamRF View Post
If you had the ability to switch jobs inside a duty, you could have taken matters into your own hand and switch to a healer/tank to carry the group forward instead of having to abandon the duty.
Still not worth the drama and headaches and problems this would bring.