Like Jholley, I firmly believe that if your going to have a FB he better be able to do more than just block. I want Mike or the ISLB worrying about the rampage, I need that Dive or quick hitter to work so that the other stuff works better. IMHO you loose a dimension if all they are is a blocker. Even if its 3 or 4 yards, it makes the other stuff you run more apt to pop!
But I'm a crazy old fool who likes the Wishbone and thought Larry Czonka was a collision looking for some one to hit! Sadly, with the advent of no touchy defense, the Power FB is a lost art.