In light of all this talk about most attacks not needing mitigation I would like to mention a few things.

Preys in a12s are, at this moment, nearly impossible to mitigate without mitigation. Raw healing is simply not enough to keep you up.

Same with most mega holy's. Some dps will straight out drop dead without shields. Something a WHM simply can achieve on everyone time, but an AST and SCH can.

I would also like to bring up the knockbacks in sophia ex. If your shields are strong enough, your character will completely mitigate the knockbacks entirely. Once again, something a WHM simply cannot achieve, but an AST and SCH can.

The advantages between high mitigation and high healing are not comparable. High mitigation simply does so much more.