It's not, though. You can even take it to an extreme and look at the dumbest possible thing he could do. Down by one run, 9th inning, bases loaded, two out. Vladdy's scheduled to come up. Montoyo decides to pinch-hit Tapia instead. All else being equal, the chances of bringing that run in go from something like 40% to about 30%. That's a big difference, but it's also kind of not, especially considering it's so extreme that it's something no manager (including Montoyo) would ever do.
The things we complain about the most are tiny little things that feel bigger than they are but realistically have extremely minimal impact on the outcome of any play, let alone an entire game.