I actually went reading through a bit about Dark Knights in various Final Fantasy games. On the whole most DKs don't have many HP-draining abilities (usually two at most), though some had more then that. Bravely Default actually seems to have had the most amount of such. XI also seems to be a bit higher having three from what I see either draining-HP or being dependant on damage taken. They also had a move that decreased their defense for more attack.

While XIV DK doesn't have any HP-Sacrificing moves, nothing says the devs might not add them in at some point. The more levels we get the more one SHOULD be better at their job as well as those around them, plus given how much we see healers talk about how healing by itself isn't that bad/too easy something like Darkness could be seen as a way to keep them on their toes as well...so long as it has the potency to make up for the loss of Healer DPS in the eyes of the player base.

As an aside, it would probably be a good idea to stop snipping/name-calling each other. Noone is going to see eye-to-eye when everyone is being aggressive against each other.