Ideally, you don't use 32 year-old Jody Mercer at all because he was a poor fielder at that point.
There have been platoons in every era since I have been watching the game. They are harder to do now because so many roster spots are used for relievers but, as you pointed out, teams are still using them. I don't know if I like an overabundance of platooning. If someone is a good fielder, they shouldn't necessarily be platooned because their defense may override the value of the offensive platoon. An old timer example would be Lou Whitaker who never hit well against LHP but his defensive value in his prime was more important than an offensive platoon advantage. Later in his career, when his defense slipped, it made sense to platoon him and they did.