Interestingly, I've recently been going thru my stuff, rechecking/confirming how much ammo I have on hand for each of the various guns. Got more 30-30 than I thought, but not enough 30-06.
I've previously made a couple "kits" that may come in handy.
Buying ammo for calibers I don't have doesn't make as much sense to me as buying more 30-06 or .35 Remington, etc, as I have a current, actual need for it.
If I were to buy calibers I don't have, it would not be in case I inexplicably find someone at my door with a gun, but has no ammo for it. No, it would be for MY use, in case I came across a gun that took it.
There's an argument to be made for standardizing on a few popular calibers, but there's also one to be made for having a wide variety of calibers: you can use almost any ammo you find.
Giving ammo to someone else can open you up for legal problems. Their gun jams on your ammo, they die, family sues. Or their defective gun blows up shooting your "alleged" factory ammo, they sue.
IF I felt compelled to help someone in this way, it would be a trusted friend or family member using a gun I'm familiar with. But I'd rather they use the ammo that's been proven in the gun I also lend them.