This IS kind of late, but:
Admittedly I'm not thinking of full "I'm a warrior who decides he wants to be a frail mage now", which admittedly IS ridiculous in all but the least story focused games (unless they built it around this possibility like, say, that hypothetical Dr. Who RPG), but more along the lines of what Evillore said:
I'm thinking more that, say, I'm playing a warrior, but I completely botched it by trying to do everything, and instead respec that warrior to be a pure tank. That makes ENOUGH sense to me, especially if it comes with a price like that, or simply slap the gameplay penalty of lost levels like Etrian Odyssey did so you can come up with your own idea of why s/he's suddenly much weaker but more specialized (I'd say forcing yourself to fight in a new manner inherently means you'll be less competent for awhile). This is more along the lines of an emergency out button anyway, one you can't take lightly, and if you're not going to make gameplay a complete pushover or at least allow you to switch difficulty to "narrative' on a whim or whatever then it would be a solid compromise for those players that screwed up. That, and the whole Souls thing seems to imply this could actually make enough sense RP wise anyway if you want to be really anal about it.
