This is it right here! +1If I was part of a squad and had a platoon backing me up, along with the resupply capabilities of the US Marine Corp, I would be less concerned about having back up sights in the event of an optic failure. Unfortunately, I do not have that level of support.
For civilians, it's not even so much a worry about batteries dying or an optic failing in the moment. If SHTF, how difficult will it be to procure a new one? Basic supply chain disruptions can make the prospect of getting one from a retailer difficult. Even if you have a backup at home, who is to say you'll be near your home when it fails.? A backup in the safe is useless if you've been run out of your neighborhood and can't get to it.
Also, it's not just about the optic failing. There are other factors that can make it inoperable. Your rifle could fall off the back of a truck. If you find yourself in an actual gunfight, God forbid, it could get shot and damaged. There many scenarios that would lead one to need backup sights. Why not keep them on the rifle. They weigh basically nothing, and the cost is not so prohibitive that I can't afford to slap some on the gun as an insurance policy.
I hope we never get to that point in this country but it's not looking good.
A spare bolt and grease is another great thing to keep in your grip or stock compartments. And a mini snickers bar so you dont turn into rosie o'donnell