Well say you have the lineup like this:
Reyes
Martin
Bautista
EE
Donaldson
Smoak/Valencia
Dirks/Mayberry
Pompey
Goins/Izturis
And come the 6th inning the other team brings a LHP out of the pen with 0 outs to face Smoak, Dirks, Pompey and Izturis. If you bring in the opposite platoonmate for each of those hitters then the opposing team will have a huge advantage later in the game when they can easily bring in a RHP to face Valencia/Mayberry/Izturis and also Donaldson because he is much worse against RHP. This will also be in the 9th inning which will severely limit your chance of scoring runs.
This scenario happened more than a few times last year and Gibby really had no options but to sit and watch guys like Tolleson fail miserably vs RHP.
My suggestion would be to keep MAyberry because he has a much better track record of hitting LHP while not relying on a huge BABIP and he can also play 1B as well as LF.