Beni in games started by LH pitchers:
.231/.273/.538 .811 OPS
Even if the starter is a lefty, the starter usually only goes 5-6 innings.
You don't really have the option of platooning/pinch-hitting at will, so you have to start Beni against lefties. He will get some PA's in those games against RH relievers and overall, his numbers will be acceptable in those games.
He has a 3.0 fWAR less than halfway through the season. He's not someone to be concerned about.