I'm very relevant to this thread because I own and shoot three battle rifles and three assault rifles every single week. I switch the guns up every day, shooting a different one for each weekday and the same one both weekend days, if I feel like doing some extra shooting I might overlap, but not miss practice for the scheduled firearm for that day. I'm right up OP's line of question, so I'll tell you why and what I've learned.
Part of it is owning a nice collection. I have the Cold War in my safe, I've got a bit of history and fun, owning nice things I enjoy. The fun part is hard to deny. Also historical, someone can come up to my property and get a runthrough of the weapons that made the Cold War (and today). Functionally, I get to train with the major common weapons on the modern battlefield, even if I'm never a super master with my AKM I'm familiar and stay sharp enough if I'm forced to use one I can with competency. I'm not the biggest fan of the AR-15, but I shoot my M-16 clone enough that I'm proficient. If I have to take an M-14 or M1a during an emergency that's not my own, I know exactly how to use it. Same thing with a FAL. My delayed roller blowback are my primary, I practice with them the most to stay at my best. In any case, this practice and ownership means in an emergency situation where I can't get my own guns, I can use whatever is available.
As far as militia goes, I agree one should own several of the same gun, so my diverse collection isn't as good for that. Certainly I could arm my very good friends during a crisis, with no interchangable parts, but I do have the number of high end guns to do it. Keeping a diverse collection is hurting your militia possibilities, but keeping a diverse collection is keeping in good faith that you aren't up to anything. Which an old fashioned honorable man like me still believes in. Owning 20 of any particular combat rifle will get you on a list for certain and will raise suspicion.
The downsides? I may shoot my G3 and C93 two days in a row, but the rest are switched up day to day for the schedule. This leads to major shakeups in your shooting, different cheek welds, different sights, different trigger pull. You get trigger confusion, you might get into the habit of using the wrong cheek weld because of the gun you shot yesterday or the gun you are best with. Its a major challenge onto itself. Trying to stay proficient with many guns makes it hard to stay great with fewer guns or one gun. The other part is ammunition and money, you end up spreading a lot of rounds over a lot of guns which means less practice with each. The struggle is real. The arguments against it are valid and have merit.
Having a collection and enjoying shooting doesnt' mean you can't train and get good at it. Certainly there are issues of time and resource spread, which I agree with. I may even change my current shooting schedule back to my main rifles and an extra. But there are benefits as well, you learn to become a good general shooter, you learn to be good enough with weapons you may be forced to use. Plus, a collection that doesnt' get shot much and is taken care of will last prehaps hundreds of years, buy it now shoot it whenever. Its never a bad buy unless you are broke and can't afford it. Otherwise it can sit for a decade before you can afford and choose to shoot it.