I have a Corbin on my bike. Had a Corbin Gunfighter on my last bike. The aftermarket seat was absolutely necessary on my last bike, but definitely not on the Busa. The only down side to the Gunfighter was that it locked you into one position, something the Corbin for the Busa doesn't do. You can still move around pretty well, though not quite as well as the stocker. The Corbin for the Busa also crowds the jewels a little in the front where it meets the tank. The Corbin takes at least 1000 miles to break in.
Having said that, I still prefer the Corbin because on any trip over 200 miles in one day it is far more comfortable, and the farther I ride the bigger the difference. Also there is the minor matter of appearance. It's subjective, but the Corbin's appearance puts the stocker on the trailer.
The Corbin isn't really necessary,but then most of the stuff we put on our Busas isn't, either. It is good value for the money, though if it happens to fit your needs.