I don't really disagree with you as much as you make it sound. I think that in general, if Hurdle wants to get Mercer in a game, he should do by moving having Ramirez sit and Kang at 3B. I just don't have as much of a problem with in a game that we should pretty much be able to win on autopilot like tonight. It is possible that there's more to it than simply Hurdle's whim/preference for Mercer.
But I think that generally, Mercer should basically play when Morton is starting and maybe one other game a week, especially if it's a LHP. I think it is also possible that Hurdle's overall strategy is something like what I said earlier, i.e. basically giving Ramirez rest days but trying to play him as much as possible besides that, in which case I think that's a bad idea.
For tonight I just don't really have as much of an issue with it. Besides the Cubs-Dodgers back-to-back, there aren't really any big clusters of tough games left on the schedule, so I'm fine with erring on the side of resting Kang a little more than usually called for given that he's in new territory right now.