It actually is. A player's defensive ability isnabsolutely taken into account when deciding whether to platoon them. An elite CF like Inciarte would have to OPS about .500 against LHP before I'd want him out of the lineup.
While .500 is a silly number obviously made to up prove your point, I understand what you mean. I still think a player like Rajai Davis would be an ideal bench player for the 2017 team, even if he just gives Inciarte and Markakis occasional days off vs LHers and is the late inning defensive replacement for Kemp. Maybe not a straight platoon, but if you are going to rest Inciarte anyways, rest him vs a LHer.