Matt Joyce is among the AL leaders in WAR -- virtually on par with anyone in baseball not named Jose Bautista -- and he's doing it all with one hand. Well, against pitchers of one hand, anyway. Joyce is raking righties. For the year, he is batting .397/.455/.690 against RHP. Some of this is luck, as his BABIP is .443, but Joyce has a career OPS+ of .929 vs. RHP.
Now, the question: Will Joyce ever be able to hit lefties, or should he be platoon only? In his career, he has 658 PAs vs. RHP and 84 PAs vs. LHP. His career line against southpaws is .167/.274/.264.
Joyce won't turn 27 until August, so he is just entering his prime years. If he could hit for power against lefties, which he hasn't in his limited chances so far, it would probably be worth letting him go to the plate. He certainly will not get better by sitting the bench. On the other hand, the opportunities against RHP are plentiful, so he is a valuable weapon. Maybe it's better not to mess up his head with trying to hit lefties.
When Chase Utley came up, it was thought he wouldn't be able to hit lefties. After a year or two of some struggles, he started to mash them. I guess part of it depends if you have a suitable platoon partner. Otherwise, I guess it's best to give Joyce his chances and find out.