Fair point. The Brewers, by wOBA, have been much better against RHP (4th in MLB at .368) than LHP (18th in MLB at .322).
Upon further reflection, you may have me convinced. The Brewers have gone with Brosseau/Miller (w/Anderson in RF and Wiemer in CF) in favor of simply playing Mitchell and Turang against lefties. And beyond the hitting platoon, playing Mitchell and Turang would be a superior defensive scheme.
I would maintain, though, that the Brewers need to find out if Voit, Brosseau, and Miller can be counted on to mash lefties. If they don't play, we won't know. If they CAN'T hit lefties, than they have extremely limited value to the team, and the team will need to look at making changes, because it would be unfair to simply throw Mitchell and Turang to the wolves as rookies, and NEED them to produce in that role. Voit, Brosseau, and Miller need to lefties THIS season. There is plenty of time to find out if Turang and Mitchell can hit lefties.