Let's see. According to the description, the HP expenditure occurs specifically when using Dark Arts. The abilities that use DA are: Power Slash, Souleater, Dark Passenger, Carve and Spit, Abyssal Drain, Dark Dance, and Dark Mind. For the rest of your skillset, using Darkerside is a dps gain at no additional cost over regular Darkside.

If you want to play defensively and still use it while tanking, in single target you'd probably burn as much MP as possible on DAC+S and DASE over about a 20 second window, once per minute. Once your MP was sufficiently low, you would use the rest of your skills under the new ability, without DA, until it's nearly time for the next DAC+S.

This is effectively a Berserk effect. Without pacification. On a 40 second recast. For essentially free.

The most interesting side-effect of all this is DA Abyssal Drain. The DA effect converts all of the damage done back into HP. So if you chose to spend HP on it, you could spend HP to use the skill, and then potentially gain it all back. Infinite Abyssal Drains.

I seem to have suddenly misplaced Grit on my hotbars.