Reasons to have 10K (or some other large number) rounds of ammo:
1) If the SHTF, you can always trade ammo for food, gas, bullion, etc. If that happens, make sure that the tradee doesn't have the gun that shoots that ammo on or near his person, and that you DO have a loaded gun ON your person (yes, Virginia, I remember the gunshop scene from Terminator 1)
2) Cheaper by the boatload; also, as many above pointed out, prices rise over time, and getting bargains (esp. for surplus) is a decent long term investment.
3) It may not be available in the future. Some ammo that I have was already outlawed from importation (steel-cored Chinese 7.62x39), discontinued (Black Talon) or running out (.30-06 AP). Who's to say that we won't get laws like Mexico or many other countries, which virtually prohibit civilians from owning guns that shoot military calibers? Who's to say that some President won't declare a state of emergency and close all of the gun shops, indefinitely? How about a state of emergency where all new ammo production is for military or police use only (as in a war with a very populous enemy like the Chinese)?
3A) For the same reason, I have plenty of components on hand.
4) Being Jewish, I don't intend for myself or anyone in my family to be stuck with nothing, or even with only a few rusty old revolvers or rifles and (literally) a handfull of ammunition, as were the Jews of Europe in my grandparents' day. Nope, this ain't gonna happen, especially not after the Islamofascist bastages lopped off the head of yet another Jew this week, because they'd like to bring some or a lot of that to our shores. They'll only get me when the barrels of my guns melt, and it takes a lot of ammo to make that happen.
5) If the SHTF and you have, say, 1 dozen guns, you are able to give/lend a gun and a plentiful amount of ammo to friends and/or family. While I'd probably do this, the antis among them wouldn't get anything from me except "But I thought that guns weren't good for anything except killing people" or "But what do you NEEEEEEED a gun for?" or "You did everything you could to take this stuff away from me when it was widely available for you to buy, and now you expect me to GIVE it to you?"
6) Its fun to see the
look on someone's face when they figure out that you could start a revolution in most South American countries just with what's in one room of your house.