Big muzzle blast and flash. And when you're tired of that, you slap on a suppressor for the cool factor. They're also pretty handy. Otherwise, yeah, you lose ballistic performance.
Personally, I like longer barrels. I've been surprised that some stubbies perform as well as they do, but I prefer 18"+. The XCR at 16" is as short as I care to go. It's also why I like bullpups. Then you don't give up much to stubs in OAL, but you still have a longer barrel.