How is that going to change anything for the variability, considering an active tank usually has roughly the same ap everytime he tanks? And aren't you "punishing" tanks for mitigating properly with their CDs? Same could be said about DS as it is now, but if you include AP, you're doing the same thing twice. Come to think of it, the whole concept is already there with the incoming damage component.
Edit: Oh, wait... I forgot nowadays every tank mistakes himself for a dpser. Nevermind, scale it with AP, it'll do "moar dps" that way. And that's what tanking is all about these days, innit?