I can certainly see why DHam should not be playing much, except as a late inning defensive replacement or PR'er, but I see no reasons for Grissom to be playing on a team in contention.
Sure, he may end up hitting significantly better than DHam, but I don't see why we should assume that, just because DHam had been around .400-.500, most of the season. I don't think he is that bad and will continue under .500. I'm not sure Grissom will hit more than .600 or .650.