you'd be causing your engine to either run rich or run lean. Basically, the engine will be working harder or not working hard enough, and both are bad for the engine
not re-jetting could seriously harm the life of your engine. You don't have to do it, but obviously if you don't re-jet it will cause some serious problems
you also may not get it right the first time you re-jet and may have to make adjustments every so often. Most people never get it right. That's the main reason I've stayed away from aftermarket exhausts. Plus I like my bike quiet lol
Agreed, I like loud...that's what my truck is for...my 500 had pretty good sound for stock, but even that was too loud for my liking, needed combat earplugs to be comfortable. A nice quiet motorcycle exhaust is good for me. Besides it makes enough noise when I am up in to revs to satisfy my noise needs.