Hanigan is pretty damn good. He's gold with the pitchers, has a great OBP, and is great at throwing out runners.
You can't underestimate how much the pitchers like throwing to Hanigan. The guy is a great defensive catcher, and if there's one position you want that kind of excellence at, it's catcher. I think Baker and Jocketty really value Hanigan and are not inclined to reduce his playing time in order to develop Mesoraco.
That said, I agree with what some others have said -- if Mes can hit when given the opportunity (and be solid behind the plate), he may be able to work his way into more time than he got last year.
To my eyes, Mes didn't look great at the plate last year, primarily because he seemed overanxious, jumping at the ball. Poor balance, especially on balls away, where you'd see him fall towards the plate after swinging. He's got to find that center of gravity again and lock down his lower half. If he's able to do it, he can be a very dangerous hitter. But I am not assuming it will happen. Got to see it first.