I see the wisdom in what you are saying bodie, and I am a big detractor of those that manage "robotically" as you so well put it. I've been mulling this one over the past day or so, and I was about to side with your argument until I read this:
True, but then you will force your setup guy to pitch two innings (which isnt the norm) and then the next day your in the same situation, but your set up guy pitched 2 innings the day before. Then you have to use your closer for 2 innings. When only pitching 1 inning a game it allows the pitchers to stay fresher and pitch 3 or 3 games in a row if need be.
I hadn't even thought of what Magnet is saying, but I have to agree that it is a convincing argument for doing things "by the book". If you are going to have a specific setup man and closer, I think you have to do it by the book and use one for the 8th and one for the 9th. In cases of teams that don't have a specific set-up man (of course very rare nowadays), then I can see extending the setup man for an extra inning, because you can come back the next day with a different setup man.
If you overextend the setup man and he can't pitch the next day, then you risk the scenario of either having a middle reliever working into or for the 8th, or you have your closer pitch 2 innings (as Magnet said) overextending him, and then you lose him for the next game. The fast-paced game after game scheduling of the season dictates that you sparingly use your best relievers so you can count on them to be in every game if need be.