I too don't think the MP consumption will constitute Pie builds. I think MP management will be a limited MP pool that will work as a priority system during fights.. Like Vargas said about stance dancing
I imagine there's going to be MP consuming abilities that do each of the following

-Damage mitigation, Damage boost, Threat Generation
Tanking will involve managing these options (limited by MP pool), deciding when it's safe to switch from threat generation to damage mitigation or dps boost. Burning too much mp in 1 area would handicap other options, so focusing entirely on threat or mitigation would drastically reduce dps potential and vice versa.

When they said "MP to use powers of Darkness" gives me the impression they'll get their own darknessy version of Flash, GLA may not be a cross class??