"Gabe Gross versus righties: 125 plate appearances .269/.368/.398
Gabe Kapler versus lefties: 72 plate appearances .260/.333/.520
Both have gotten at-bats against their same-handed brethren as well. Gross has about an extra 20 at-bats, but otherwise the pair has combined to be something like a ~.250/.350/.450 right fielder with above average defense to the tune of about seven runs.
That's Jayson Werth, my favorite Matt Joyce comparison.
Even if Joyce were Werth from day one, it's basically a zero sum move at this point -- unless you make the point that having Gross/Kapler as a fourth outfielder everyday instead of every other day provides an upgrade."
While Kapler and Gross are cheap, there is the cost of a roster space. But a platoon is a much better use of a space than a pinch runner, defensive replacement, or third catcher. When your team has a hole, root for a platoon, especially if the two platooners share the same first name.