Interesting points, and to be fair the books did not have dirty bullets, and they would end up shooting away their money, quite a lot of it on occasion, although then it was more a case of 'if i do not use these bullets now i won't have to worry about affording food'.
There are some serious issues with this however. For one thing, and this occurred to me when thinking about the bottle caps in fallout, you have to count every single one of them, and in both games there are some really expensive things to buy, we're talking stuff that costs thousands of bullets/caps, you have to count every single one of them. Not to mention the fact that bullets are somewhat heavy.
Granted it might be easier with bullets since you can use the different calibers to work like real life paper (9mm bullets as pennies while a 5.56 would a 10 dollar bill, or something) but the thing is, money works because it's useless paper, and it's value is predetermined. Bullets have quite important uses besides being currency, and it's use my vary depending on what the other guy needs, you can have a box full of 12.7mm (.50 cal for the US among you) ammo but it will be useless to the other guy if he doesn't have a weapon that can use that ammo, unless he is a trader. Then there's also the rare ammunition, either because it's specialized (subsonic VSV ammo, or underwater bullets for the underwater rifle) or rare because it isn't found around there (for ex NATO 5.56 in russia where they use 5.45).
The value would also be determined by how common one type of ammo is, in some cases 9mm might be more expensive then 5.45 which in turn might be more expensive then older 7.62 simply because they are harder to find (for ex: most of the 5.45 was on the surface and in use when the nukes struck while the the 7.62 were in reserve storage). And that's not counting the possibility of market crashes every time someone finds an abandoned ammo cache full of bullets.
On another note using dirty bullets (which personally i believe are hand made with crappier metal and a lot less gunpowder) actually prevents the economy from crashing during the game. Think about it: If everyone carries currency to shoot then obviously you will be rich really fast, in the game, by simply head shooting or mele killing your enemies, and then looting all the bullets they haven't been able to fire, at least until you have to use that on the obligatory bullet sponge monsters. Using crappy non-currency bullets balances that by giving the enemy something to shoot you with as well as creating a shortage of currency bullets, after all if bandits had currency bullets they wouldn't need to be bandits now would they?
Sadly it's not without it's issues and frankly it will still eventually boiled down to a barter system anyway, most people do not have access to bullets, especially in the beginning or for various reason, in which case it's better to ignore the bullets as currency and focus on the barter aspect (like STALKER did), which again boils down to need versus have. After all, anyone know that a knife is worth an ak-47 if there are no bullets to be found and you're expecting danger to come knocking.