There is a related post under Engines - Factory dual exhausts on a 250
I think the last post on it was 7/16/09
I realize it is not the same question you are asking, it is just a related topic.
I agree with Hank, a lot of it has to do with the sound.
There is also some math involved to make sure you have a certain amount of back pressure. You can find places online to study that, but some back pressure is good.