I don't think catchers "vary so significantly defensively". Absent any reason to believe otherwise, I suspect they probably vary about as much as any other position. The reason is because of the weight placed on the defensive ability of catchers. You see teams getting away with crappy fielders in other positions because that's where you end up with more offensive assets. Where are the Alfonso Sorianos and Ryan Brauns and Adam Dunns and Prince Fielders of the catching world? There's much less variation than the other positions. That doesn't mean all catchers are practically the same defensively, but it's unusual to find the scope of variation between catchers as you do between different infielders or outfielders. As an example, and this may not be a perfect comparison, but I'll throw it out anyway. Mike Piazza was the biggest disaster I can remember, defensively, from a catcher and he was light years ahead of, relatively speaking, a bad LF/1B/3B etc. People were willing to deal with his bad defense because he was probably the best hitting catcher ever. He was much better at his position than the LF version of Adam Dunn or 2B version of Alfonso Soriano were in relation to average or above average defenders were at their positions. When Mike Piazza's defense is an extreme outlier, that shows the difference between catchers defense and every other position. Good point.