Skorski,
Lower EGT's and a nice diesel sound is the main reason. We even have a new high flow muffler assembly that uses the factory intermediate pipe tubing and replaces the muffler and tail pipe so the muffler assembly fits correctly. It is our Spiral Flow muffler and tail pipe set up to connect to the factory intermediate pipe, not welding a muffler in at an angle or any of that, just cut the factory tube as itenters the mufler and bolt it on. It's made out of T-409 stainelss so it's a similiar material to the stock system. They are available on a drop ship basis for a while but we have it if you want one.
Mark @ DPPI