I'll agree with that. However it is literally one piece of gear (feet). Every other slot for "BiS" has PIE on it as it is. I forget how much MP = 1 PIE. Google is saying 8, so 16 PIE = ~128 MP, which = ~+25 Aetherflow and whatever that does to MP regen ticks. Worth it over 21 crit? Probably, and I've edited that list now to reflect inconclusiveness.
Besides who knows how grindy it'll be to get to rank 30.