Actually it's only Warriors who really have a penalty. Dark Knights get NO penalty switching out of their tank stances. And Paladin stance swaps have no effect on their overall damage - just a temporary setback to their utility.

The only job that really suffers from this are Warriors. As for why they did it, yeah... it probably IS to encourage people to stay in tank stance while they're tanking. The people in charge of this change were probably healers who got fed up of having to take care of Warriors in Deliverance, so they decided to screw the job over and make sure this doesn't happen as often.