I reckon an AR15 pistol might work for some folks as a "rule-booker" weapon that could share magazines with a companion AR15 carbine, as some jurisdictions do not allow loaded long guns inside vehicles, but do allow loaded pistols. Under such conditions, I would want a quite long barrel, a Noveske flash hider that diverts flash and blast forward, and a buffer tube to act as a third point of contact. Yes, such a weapon would primarily be the range toy, of the two, but could have a loaded magazine in place during transport.
I see a conventional pistol, that can be holstered, as much more versatile and dynamic in a wider range of conditions than an AR15 carbine, so I would not forego the holster pistol if toting an AR15 pistol.
Thankfully, I live in Texas, where our long guns can be loaded while inside vehicles, concealed, or not,
so a minimal-length carbine makes more sense as a fighting weapon. I may well, however, buy a designated pistol AR receiver, build it with parts on hand, and purchase a pistol-length upper, install an optic I have on hand, and have a nice range toy; nothing wrong with that!