I think one of AB's greatest shortcomings was a perfect fault for how Ben Roethlisberger played the game.
I do believe that for a good part of his career Ben either wasn't accurate enough or "quick-reading" enough to just "pull the trigger" on a pass play as it was drawn up. I think a LOT of Ben's "run around and make something happen" occurred because Ben either missed the open read as it happened, or he didn't feel he could get the ball where he needed it to be in the moment. Therefore, he would hold the ball and start looking for something to open up better.
AB happened to be a guy who had NO PROBLEM whatsoever breaking off a route and going to open space. If Ben hesitated on a throw and it didn't come out, my bet was that AB instinctively started working toward open space to give Ben a throw he liked.
Those things worked together beautifully. But if you've got a guy like Brees or Brady who are going to run the play as it's drawn up unless **** completely falls apart, AB isn't nearly the asset he was with more of a freelancer like Ben.
In the end, out of all the receivers I truly believe Ben made better, I think it can be argued that Ben and AB truly made eachother better.