Pitch calling will help/hurt a pitcher a certain amount. It's not a drastic amount because pitchers can call their own pitches. A bad catcher will hurt the pitcher a bit because he will irritate the pitcher with shake offs, calling bad pitches, etc. A good catcher will help because he maintains the flow and does a great deal of the mental work for the pitcher.
That said, every pitcher at the big league level has been pitching for years (amateur and professional). They can call their own game, so a catcher with no pitch calling skills isn't going to cause them to get shelled. Over the long haul, having a catcher with a low pitch calling rating is going to cost his staff .005 - .015 points in batting average allowed which is very significant.