Maybe, but having a 6 man rotation will subtract 1 arm from the bullpen. In the event that one of our starters can't get out of the first inning, we'd be even more behind the eight ball in that game than with a 5 man rotation.
Plus, I would be totally against a 6 man rotation just on the fact that our starters have a routine with 4 days in between starts. Adding an extra day could have detrimental effects.