I should know the answer to your question but honestly, I'm not sure. I don't believe the muzzle device added to the barrel count as barrel length. I could be wrong but that's my belief.
For AR's with less than 16" of barrel, check the fine print. Decent chance the butt stock isn't a true butt stock (rather a pistol stock technically) or, when you purchase it, you will be advised of the need to get a tax stamp ($200 btw).
Shorter barrel lengths are useful in certain tactical situations. For example, and this is my reason for needing it, clearing a building or heavy brush/wooded area, is easier with a shorter barrel. With a longer barrel and gun, getting through doorways is more difficult and cumbersome. Add to that your teammates, all of your gear and clutter in a crack heads home.
Perhaps most importantly, when you clear a doorway for example, if your barrel goes through before you do, that gives the bad guy an opportunity to grab that long a$$ barrel and now you're fighting for control of the weapon. Bad day.