I've heard that said on these board before about Fitz being a curse. My response has been that, hey, we're living in a post-drought world, so all bets are off concerning curses!
I would like McCown as a QB2, too, but I'm not sure he would want that role. He could very well go somewhere that ensures him at least the opportunity to battle for QB1.
Curse aside, I think Fitz would be at least a top 10 back-up QB in the league, if not top 5.