The500man
I can sympathise with your feeling that the ATPL theory syllabus has a lot of irrelevant stuff in it. However you aren't completely right. There is some pretty useless stuff and yes impedance is probably up there (but only 6 questions on electrics feature in an AGK exam).
On the other hand there's a lot of stuff that is very useful (or might become useful when faced with an unanticipated emergency). The problem is that it feels irrelevant because so few theory notes take the time to connect theory with practice.
I think that'll change soon (and is indeed already changing) because airlines are demanding a better pilot product.