Q:

Has any baseball team used a "platoon" for fifth starters?

A pitcher can be either left- or right handed, but the better ones pitch reasonably well to both left and right handed batters. When you get to the fifth starter on the roster, however, many teams are dealing with marginal throwers.
Some bating teams have relatively more of left-or right-handed batters. So use one pitcher against the one set of teams and the other pitcher against the other set of teams. Jeff Locke and Brandon Cumpton of Pittsburgh come to mind. One is a lefty and one is a righty.
Are there any teams that in fact, do this, because they have "fifth" starters on the roster who throw a lot better against either left- or right handed batters? Perhaps combined with another back end starter who has "platoon" splits the other way? Meaning that it would make sense to "platoon" a "backend" starter role so that the tandem may actually be better than a fifth starter?

A:

During the 2008 season, the Milwaukee Brewers used a platoon for the fifth starter position between Dave Bush and Seth McClung, following the mid-season acquisition of CC Sabathia.
info: http://m.brewers.mlb.com/news/article/3149657/

McClung gets the nod on the road, Bush at home. With McClung preparing for his start, Bush was available Friday out of the bullpen. McClung will shift to the long relief role in the middle of next week.

The statistical splits provided evidence for this uncommon approach, as Dave Bush's ERA at home was 2.49 and almost 7 on the road. Conversely, McClung's ERA on the road was 3.28 and almost 5 at home.
While Milwaukee made the playoffs that season, it was largely due to CC Sabathia going 11-2 with 7 complete games after joining the Brewers, not due to the 5th-starter platoon.

