Jason Castro is a solid defensive catcher, but nothing amazing. Frankly, Drew Butera was better defensively. Thing is, both Garver and Astudillo appear to be equally solid behind the plate, and both those guys are much better hitters.
Unless Castro's bat bursts into the incandescent flame of justice and vindication, I don't see much reason to keep him around over the two demonstrably better hitters. If Castro were gone and one of the other guys got injured, I would not hesitate to bring up Brian Navarro, a big, strong defensive catcher. And I hear he works relatively cheap.
Navarreto has a .573 milb OPS.
To put that into perspective, Drew Butera had a .610 milb OPS.
Jason Castro had an .804 milb OPS.
With all the hyperbole about how awful Castro is with the bat, I find it fascinating that people want to replace it with someone who will hit much much worse.