For subsonic applications with a suppressor, (which most have no need for unless you're a mall ninja) they're used by guys hog hunting at night...
Variety is the spice of life!
I want one just because I can. Was gonna build a Whisper but there isn't much difference performance wise between the two and there seems to be more aftermarket stuff coming out for the AAC. I'm a reloader too so the round is more attractive to me since I don't have to pay for production ammo.