Where I live in Kansas, there are wide open spaces. My nearest neighbor is a mile away. I could conceiveably start engaging a target 1000 yards away in any direction with my scoped K31 or my scoped Enfield #4 Mk1. A little closer I could start using my Garand or HK91. If targets get within 400 yards or so, then I can engage with my AK or AR. Yes, these are in a variety of calibre's but who knows what my resupply might be. You may love the AR but if .223 isn't available at the time you need it..... I am pretty much able to use any major calibre that might come my way.
Also, in a SHTF engagement, you can't play the same game as your opponent. He may be more accurate and stronger than you. You've got to be prepared to engage in a variety of ways to counter an attack. If someone is coming at you with AR type rifles, they are not going to be able to contend with long distance accurate fire in open spaces. If they are engaging you at long distances, you are going to have to move and engage them with a flanking manuever while pinning them down with your own long distance fire. Having one type of rifle will not make you able to contend with, in a significant way, a variety of situations.
If I were on the move always and could only carry one rifle, it would be my HK91. I would avoid conflict at all costs but if I had to, I would have a powerful weapon that could engage at a variety of distances. If I needed more than the 160 rounds I could carry, then I failed at avoiding major contact with hostiles.