It’s not the mock-up that’s the problem, it’s the approach AFTER the mock-up is done.
It’s the approach before the mock-up as well. A mock-up is not a finished product. It’s …well, a mock-up.
Oh and for the record… I used to mock-up EVERYTHING in Photoshop. Just lost the taste for it, but that’s just my personal style.
I started out doing so also. I never found it useful. It was pretty much a waste of time, in retrospect; it was actually hindering in some ways.
But, again, different people do things in different ways.