Falling into a number of traps here..
I really don't see a reason for enemies carrying money.
Trap #1: Not everything in the game needs to have pure advantage for the player. Enemies can carry their money because they like being rich, and the best way to make sure they STAY rich is never letting their money out of their sight. (Ed) This is a bit of game flavour which makes perfect sense - AND it does, in fact, have potential use in game; read on. (/Ed)
In fortress mode, you have a set amount of coins in the economy. Adding more isn't really necessary. It's even better to not use coins at all due to the constant carrying.
Trap #2: The economy as it stands is known to be flawed. The economy will not be broken forever. Rejecting suggestions because they don't work with today's flawed implementation isn't the wisest of things. (It's far from the stupidest, but the whole point of a suggestion is to think forward.)
Once the economy(and esp. stacking of items, including coins) is working better, dwarves might well require coins and other precious goods rather than relying on omniscience about their own bank balances.
Having a way to carry money, combined with intelligent stacking(or at least storage of multiple similar items in a container, as Toady recently stated marksdwarves are doing for ammo) may in fact banish a great many of the economy's problems.
In adventure mode, each civilization has a different form of currency, don't they? So let's say you steal money from an enemy, you surely aren't going to spend their currency at their home, will you? They probably aren't fond enough of you to let you waltz in and buy that Carp Waterskin you've had your eye on.
Trap #3: Not all money is without intrinsic value. In fact, purely-extrinsic currency(which gets its value for no more reason than that the issuing government says so; paper money, but also base-metal coins) is, IIRC, a fairly modern thing; until relatively recently, the base unit of currency was based around precious metals.(ed.: e.g. the pound sterling, based on sterling silver; many currencies have been based on gold. /ed) In DF, ALL economic coin values are proportional to the value of the metal; a single coin, in terms of the economy, has a base value of 0.5, and the multipliers of x2, x20, and x30 give copper, silver, and gold their respective values of 1, 10, and 15 dwarfbucks.
You might not be able to go into that enemy city and buy their carp leather waterskin, but you could take their coins back home and add them to your own wealth to buy that *<<-naked mole dog leather waterskin->>* instead. You might not get full value for coins from another civ, but that's just because of the need to make sure they're pure.