The only ff I played was FFTA so my knowledge is quite limited. With that said though, gaining higher defensive stats for every loss of raw hp sounds exciting! Something healers will have to coordinate accordingly for sure.

From what it sounds like, SCH would synergise greatly because of relatively lower raw healing done and focus on damage prevention. And WHM could focus on stoneskin as a pseudo heal.

It would also be the biggest noobtrap though. 'Oh I get tougher the lower my hp is!' at below half hp or something.