Of course, he's not as good against RHP. But you still throw him out there as often as you can (which CC has done), not just because he's got an .801 OPS overall, which is what really matters and which is second highest on the team, but because he plays defense at a high level, which Rowdy and Keston do not.
Rowdy hasn't been hitting. Contreras is our second best overall hitter, but was unavailable to C today. Not difficult to figure out why he would DH over Rowdy. If he doesn't DH on Friday against a righty, then things obviously would appear different.