The devs were the ones who originally proposed DPSPM, though admittedly I did back it, but that was largely on the basis of differentiating the spells, as in I hadn't dwelt on it much.
That said, so far I have yet to come up with a good way of demonstrating its worth, so it's entirely possible that is has none.
ah, statistics.
On the note of Mana per second, I'd argue you'd be better off with say overall mana lost per second, something that figures in the mana regen.