Scaling a TANK's damage or ability to tank with how much health they have remaining is a bad idea. It ties you too tightly to another person knowing how your Job functions. Bad healers stand there and do jack shit until you take damage, then they just Cure 1 and continue standing there. Guess what - your Minus Strike passive will be worthless in situations like this.

The other fundamental issue with this is that this mechanic is completely worthless for the Off-Tank Dark Knight. You're in the position where you're expected to do maximum DPS and your strongest DPS gimmick is locked behind the fact that you're not tanking - and, again, relies on your healer to know about this mechanic and how it works while also knowing fight mechanics well enough to use their resources to heal you before big hits. It's silly.

Dark Knight isn't perfect, but any suggestion which makes us rely on healers more than any other tank is one that will permanently boot DRK from the meta.

(Also I just read the tooltip and assumed! That's really dumb.)