Whoever says health isn't important is very misinformed, on certain fights, for example Firebrand and Stormcaller your mitigations don't mean ****, it's all about your health
The only time that mitigation "don't mean ****" is if you're tanking Stormcaller (Firebrand is *pure* M/R so mitigation means everything there) and, at that point, your hp only matters insofar as giving your healers a reaction window to keep you alive. Unless you routinely drop to sub 4-8k hp while tanking Stormcaller, that extra hp cushion from your 26-30k hp isn't actually contributing at all. It's just sitting there, being pretty and doing nothing.