That's a good question to think about. I don't know. On one hand it does feel a bit silly to be carrying around 25,000 gold coins in my pockets. In Divine Divinity, I think I had accumulated 1,323,090 gold in my pockets by the time I entered the Wastelands. (I was planning on buying a lot of 50,000 gold charms to fill about 55 charm sockets.)
It depends on a few things, like how much weight will it take to buy a shiny new piece of (gender-neutral ;)) armour in a store? Will I become overencumbered walking back from my house or the bank to buy stuff? Will this happen often? Will the cost in gold-weight be so much that I can't even move naked to the store to buy the armour?
I don't think I like the idea of picking up a coin purse halfway through a story dungeon and becoming overencumbered.
One way to solve the weight problem might be to introduce different denominations, so instead of carrying 100 0.01 weight copper coins, I can carry 10 0.05 weight silver coins, or 1 0.10 weight gold coins. It's all the same amount of money, but the denominations will let me get more value per unit of weight. I definitely think that 0.1 weight for the smallest value coin available could be a real problem. There were things to buy in Flames of Vengeance that cost like 60,000 gold. In 0.1 terms, that's 6,000 weight units. Even 0.01 weight units, that would be 600 weight units.
Another way to deal with the wheelbarrow full of gold encumbrance problem would be storing coins in a bank (for a small fee) that allows automatic withdrawals to merchants. Storing them at your house has no fee, but no automatic withdrawls.
So, there are things you can do to mitigate the effects of money with weight, but the most important question to ask is "Will the game be more fun or less fun with weighted money?"
Gamers usually ignore the fact that money weighs nothing, but as soon as it becomes an inconvenience, they'll definitely notice it. On those merits, it's probably simpler and better to have the money remain weightless.
