Using Aroldis Chapman in the bullpen is like driving an Indy car in a go-cart race. Using Mike Leake in the starting rotation is like driving a go-cart at the Indianapolis 500. One job is much more important than the other.
It is wise to use your best soldiers for the whole battle rather than saving them to mop up the enemy stragglers after the battle is almost over. You are much more likely to win the battle if your best soldiers are fighting it.
It is wise to use your best pitchers when the outcome of the game is being fought over in the early and middle innings rather than saving them to get the last three outs when the game is already pretty much decided.
Using Aroldis Chapman for the last inning with a three run lead is a waste. You don't need a great pitcher to protect a three run lead. He should be used to make sure the Reds get a lead in the first place, not to wipe up the crumbs. Even a bad pitcher can protect a three run lead 99% of the time.
You are more likely to have a lead in the late innings if your best pitchers throw more innings. As a starter Chapman will throw 3x more innings than he would as a reliever. If he doesn't throw those extra innings then a lesser pitcher will.