I agree with your comment on Kepler being strongly out of favor with many. I felt that way at times. Wrong on Marsh.
There is a huge difference between the wisdom of patience with Marsh and Casty, and that is their widely different level of success in Philly over multiple seasons. Casty now has 3-3/4 years of playing in Philly and has received preferential treatment when he stunk but played every day. Because of that, he has a huge # of ABs as a Phillie. His record is clear: in 2424 PA -- statistically significant in anybody's book -- and put up an overal OPS+ of only 101. Not acceptable for a bad defense RF in anybody's book. His total rWAR in Philly is only +1.2 -- still a +0.3 yearly average, above replacement level -- only this season his rWAR has fallen to -0.9 -- well below replacement level. He is extremely streaky, and does have good periods with the bat, but his good periods simply don't make up for the slumps this season. At age 33, Casty shows every sign of having aged out. It's not just this season's stats. His whole 4 years in Philly have been far below his good performance prior to coming here. The Phillies are unlikely to save any significant $ on his salary and it would be beyond foolish to give him substantial playing time in the hope of increasing his trade value.
Marsh is entirely different. His rWAR in Philly is 8.3, including, including +1.0 this season, justifying more patience than he has received. In 1450 PA he has averaged an OPS+ of 113. He is a significantly better hitter than Casty and a vastly better defensive corner OF. He still can't hit LHP well, so a platoon isn't a bad approach to using him. The problem is he has gotten less than his rightful AB against RHP.
Kepler is slightly negative rWAR on the season, with OPS+ of only 87 and negative -0.7 dWAR. He hits just sort-of OK against RHP and poorly against LHP. Questioning his playing time was valid, but the Phils didn't have a better choice. He has been just OK since All-Star break after a poor start to season. I guess patience was justified.