The main reason why I'm against using a regular starter as a long reliever is because it's something that you can do in fantasy sports that doesn't help build a real team. For example, something like that would be like stacking your bullpen with 3-4 closers. That's a very common principle in fantasy baseball, but not a sound one in a redraft. In addition, in fantasy baseball people often grab the best offensive players with no thought towards salary or defense, which again, would work in fantasy baseball but not here.
The main reason why I'd be against something like using a pitcher who's SOLELY a starter as a long reliever is that it not only isn't realistic, but it would nullify any point in using their 2011 numbers as a barometer for their worth. You certainly couldn't use their WAR and assume that it'd be the same as a reliever. In my opinion, there's a HUGE difference between using someone that's gone back and forth between the rotation and bullpen (like Joba Chamberlain) as a long reliever, and using someone that's only been in the rotation (like CC Sabathia).