Measuring Catcher's Defensive Value

Trying to gauge the defensive value of catchers is a crapshoot. Take David Pinto's Probabilistic Model of Range for instance. He has the Dodger's Russell Martin listed above Jason Varitek by his method. Just saying but the rumors are the Dodgers are interested in Varitek because of the defense he'd bring to the team over Martin.

This is not a slam at Pinto's method in any way. It's more to point out that the defensive value of a catcher is almost impossible to gauge. How do you factor in a good pitching coach or a bad one for that matter? Defensive Win Shares for catchers are an even worse measuring stick.