You're right, all Snyder has is a solid arm. Oh, and a decent bat. His defense and upside aren't bad either. He's hitting .290/.373/.428 in 145 at bats this year, that's doing pretty well for a catcher. He isn't Joe Mauer by any means, but Chris Snyder is nothing to scoff at and is proving a lot of doubters wrong this year.
I don't care how young you are, major league baseball is still a very difficult grind physically and mentally to those that haven't experienced it before. Most young players have never played at this intensity this long into the year, and it does take its toll. You see the production of young rookies drop off at the end of the year all of the time because they have simply not been through this schedule before and the workload gets to them.
But if you want chastise Melvin because he is resting an injured Drew and won't exhaust all the other players by "only" playing them most of the time rather than every single day on the schedule for over a month, then by all means...