Agreed. I feel like the core of any tank's "free" sustain should be
timing-sensitive (e.g., percentile mitigation or restoration [think Death Strike]).
I don't mind forms of "flat" sustain (healing or mitigation that is not sensitive to timing in any practical sense / has little risk of overhealing or expiring before consumption) having an opportunity cost, as found on Clemency, or frequent and modular access as on Divine Might's Holy Spirit, but... just making sure you're not topped and/or are MTing before using Req phase? Ehh...
And for that to be a hefty portion of overall sustain? Or the likes of Brutal Shell, Storm's Path, and Souleater, where the healing is barely movable and barely noticeable? All that seems mostly a waste, tbh.
Yet, having said that, I'd be fine with, say, Warrior and/or Dark Knight having significant healing (something we usually consider not particularly sensitive to timing, except between two-shots)... as long as it supplied some other way of making that incentivize certain timings that required a degree of contextual awareness, etc., etc., thereby making it timing-sensitive in practice.