FFF -- You're right, of course. But some pitot/static problems will not be helped by using alternate static and I suppose that's what the Power + Attitude = Performance training is about.
I will shoot down my own argument. Imagine you have encountered significant icing conditions in an aircraft not so equipped. The static vent freezes over and you are smart enough to recognise this but you forget all about alternate static sources. With an increasingly heavily iced airframe, P + A will not equal the performance you normally get. Setting cruise power and normal attitude will lead you into problems..... and the GPS here would be a life-saver. But then you shouldn't have been there in the first place!
AA.