I think it is more complex than just benching Arraez against lefties. A guy like Arraez probably isn't good for 150 or even 145 games. Miami is seeing the fade right now and he faded last year for the Twins aided by injury. If you are going to rest Arraez for 20 games, it would be advisable to rest him against left handed pitchers. Yes, not starting him on Opening Day was beyond the pale, but playing him too much is probably counterproductive. The same with the Joe Mauers of baseball.
As for pinch-hitting with the platoon advantage, that shouldn't be for established regulars, but the Twins don't have many of them.