The way Marsh and Kepler hit LHP the best OF on a night like tonight is always going to have both Wilson and Casty. It is what it is, offense over defense, and it's not like Wilson can field either.
That Casty has cooled down at the plate is the bigger concern. You could argue for playing Kepler in RF and Marsh in LF against some RHP as the season goes on, but - as good as things are going now - if neither Bohm nor Casty are ++ RH bats (and JT isn't either) they're gonna need one.