If you remove tanks, and everyone can 'tank', everyone should be able to 'heal' too...ala gw2? Seems sort of lame. I like that you need a team to do stuff and that the team members have different jobs. Otherwise we may as well remove all classes and just have one, and three abilities, tank spell, damage spell, and heal spell.
Sounds pretty boring.
No, I think let more classes tank WAS the answer, and glyph's allowing role change was what went there with it.
Alas, that ship sailed in a frightened shiver from blizz waters, along with demo locks.