Also the cat is an effective resonator. The Brullen I installed w/o cat was way too loud, had to stick a resonator in the spot where the cat was, despite the resonator on the Brullen. I was a bit under $400 at the time and built like a tank. I think you could drive over the muffler without hurting it.
Some mufflers contain a built in resonator chamber as part of the design.
From another website:
"An exhaust resonator is used in some exhaust stystems to change the tone and timber of the exhaust sound. The resonator helps reduce the annoying vibrations of exhaust gas pulses with a length of perforated tubing inside an expansion chamber. The exhaust gasses partially escape the tubing diameter through the perforations into the expansion chamber. In the expansion chamber, the exhaust gasses expand and change tone. When the gasses re-enter the tubing, they rejoin the exhaust flow and dampen the vibrations of the combined flow, creating a more pleasing sound."
In general, you will get a more pleasant exhaust with one than without.