It would be interesting as a part of a mechanism to differentiate one of the tank classes.

I feel like both that and this will end up increasing the frustration that people have to healbot healers. "Stop keeping me at high health, you're causing more work for yourself. I take less damage at lower health." I get that it _might_ make people "in the know" more comfortable at letting their tank get low, but I feel like people that are currently "in the know" are already doing a good job at juggling damage and healing. It would also be a huge pain to balance, and would be frustrating as a tank because it's something they can't really manage.