What I don't understand is owning more than one AR-15. Isn't the point to have a modular weapon, one that you can change to suit your needs? You should be able to own one lower receiver and as many different barreled uppers that you may desire.
That's such a funny thing for somebody to say in a gun forum! Next you will say I only "need" one Glock or one 1911!
In the case of the AR... before all this craziness... lowers and parts for the lowers were cheap. I'm sure that most of us that have multiple AR's started out thinking like you. We had an AR and then we bought another complete upper. And we swapped them out and we were happy. For a few days. Then reality intrudes.
How do you store the upper that isn't on the lower? Doesn't fit right in any of my safes... way too long for a handgun section but keeps falling over propped up with rifles. So I put the lower in a soft case and tossed it in a closet. Where it was always in the way. And falling over.
Let's say the complete upper in the closet is your dedicated .22 LR trainer. So you pull it out and swap it around and practice. But if you leave it on the lower... you don't have a rifle in "ready to go" condition. So you take the .22 LR off and put it back in the closet. But you want to shoot it again that weekend.
At some point you tire of the wrong upper always being on the lower. And having an orphan upper falling over in the safe or closet. So you by a stripped lower for less than $100 and build a new one. And you realize then just how much easier it is in every way to just have a lower matched to a particular upper. The lower on an AR should match up with the upper from a configuration standpoint. A light target trigger might be appropriate for your hunting AR but not your duty one. You might actually want a fixed stock on one for some reason. Or a heavier adjustable stock on one while you would really prefer the lightest possible one on another.
So you build more lowers. Every time you end up with a new upper... someday the lower will appear. Has worked that way for me!