Defending the defensive replacement by saying you have to assume your team will get the job done is foolish. When your team loses, and the defense had nothing to do with it, you can't just say, well, I put the right guys in, they just didn't do the job. Similarly, when you sac bunt, but don't end up scoring, you can't just say, well, I did the right moves, they just didn't execute. A defensive replacement is most likely not going to make any difference defensively in the 9th inning. The odds of him making any difference may or may not be any higher than the odds of him making a similarly negative impact at the plate after the team loses the lead. The problem is people are assuming the starter will be a liablity on defense while the replacement will prevent a run. And at the same time, they are ignoring the offensive difference. You can't do that. This is the same type of conventional thinking that permeates throughtout the game. It sounds like it makes sense, but it might not actually improve your teams chances at all. Sure, there are games when the starter will let you down and a run will score. But there are also games when the lead will be lost despite making the defensive change. And you will fail to regain the lead thanks to a weakened lineup. You have to take it all into account, not just the simple notion that Player A is a better defender than Player B, therefore, I'm making the change. This doesn't even take into account the rather dubious way many people judge defensive quality, nor the possibility that enterting a game cold may negate some of a defenders advantage over the regular.