I never quite understood, why in Fallout 3 they chose to use caps? It made sense in Fallout 1, since caps were backed by the Hub (which was an economic superpower at the time), but were eventually abandoned as currency, and replaced by gold which was backed by the NCR, Vault City & New Reno.
I agree it does seem quite strange. Personally I don't think caps would have become currency - in fact I don't think there would be currency at all anymore.
It would revert back to the barter system, where Items, Food, Water, Medicine and of course Weaponry would become the currency of the day. Paper or Coin money would be meaningless, worthless garbage because that's primarily their value today (other than metals to be melted down into bullets). We add money to these games because that's what we know and it passes the Fun test (Who doesn't like to shop?). Plus the barter system is completely subjective to the transaction taking place, and people might find this strange or difficult to conform to.
Caps makes a fun currency, I think it was used in the game for the sake of money being Bottlecaps, and for no other "realistic" reason.
I'm down with that, caps are fun!