Class balance in games is just as important as the balance between time invested and rewards or status earned in an MMO. If you see a max level Panda that should stand for something. It means the player passed all challenges to reach that level as a Panda. I know some will argue those things are meaningless because Wakfu can be so easy at times. But that in itself is part of the problem.
It's hard to put into words. This service feels wrong.
As others have stated... many players will buy these to escape their poorly designed, bugged, and weak class. These players do not buy this because they seek a new challenge but because Ankama screwed up their class in some way. If this feature makes good money, what's to stop Ankama purposely "updating" random classes to start the class change merry go round.