Without trying to replicate @chpettit19s post, Castro's value is in his versatility, which is key for a bench player. That is huge for a team over the course of a long season. If he's able to provide something with the bat too, that's just gravy, in my mind. On this team, I think Castro provides more value than Kepler does. Kepler has guys waiting in the wings behind him that can play RF and hit to his level. I don't really think that there's anyone else on the 40-man that has the versatility of Castro.
Given that, I'm not sure what the point of sending him down would serve.