I have considered it. I just received a Bushmaster catalogue in the mail as of last night and inside it has several calibers and uppers listed. I haven't looked through the entire catalogue yet, but I've decided to try and see which option is most cost effective for the AR configuration.
To purchase a .223/5.56 since all (new?) Bushmaster rifles are both according to their catalogue and then get a 9mm upper. Or to get a 9mm carbine and later pick up a .223/5.56 upper/mag set.
The .22LR would be fun as well and super cost effective. I know the argument of the AR jamming has been beat to death and is usually ammo or mag related, but I wonder if this would cause the AR to jam since ammo can be finnicky. Although since we are now talking about the AR in .22Lr rather than .223, it may be a whole 'nother ballpark.
I won't be using the AR in any caliber for home defense in rust belt Cincinnati, but it would be nice to have a carbine in case things follow in the footsteps of 2001 when idiots decided to riot where I work and live. When I finally gradute, I'd like to take some sort of training with the shotgun and rifle to become more prepared for SHTF scenerios in general. Yes, I am a chairborne ranger.