So I've been reading a lot of posts talking about how DRK's are MP vacuums and I don't quite understand this.

I'm not insanely far into DRK but from what I can tell, DRK's use MP pretty scarcely. A bit for the first bit of a pull with Unleash but after that, there's no real reason to use anything that consumes MP unless your popping DPS while your tanking. Like anything involving Dark Arts is, if memory serves, a long CD of like 60s for each. Any time you aren't try to haul DPS, you probably shouldn't even have darkside on. Am I just not far enough into the class to see the use of all the MP skills or is this just the fact that people feel the need to use all the skills available to them so they make otherwise bad rotation choices?

Note: I consider a tank's job to be pretty much strictly hold aggro off of mobs unless there's some sort of mechanic the tank needs to tend to. DPS checks and the such where the extra DPS is nice.