I don't really see a reason for it until it serves a purpose. I very much appreciated how different faction's in Deadfire dealt in different currency - though of course the game didn't force players to fiddle with it. Still a nice worldbuilding detail.
I have seen anything in BG3 so far, that would support or require multicurrency trading.
The thing is, it doesn't really matter to me that BG3 doesn't require it. D&D itself likely doesn't require it. Just like how people argue over the value of the alignment system, there are things about the game that people may feel are unnecessary, but they are staples of the core mechanics and they provide a layer of depth beyond the surface that may not feel important to everyone. It's a function that is built into D&D, and as someone who loves D&D as it is, understands and appreciates its mechanics, I want BG3 to operate as closely to P&P D&D as possible. I understand that the vagaries of game development may prevent it from being implemented, but that doesn't mean I don't still want to put the suggestion out there, in the hope that it can become a reality.
If they build it properly, I see this game as a long-term platform for D&D content. If they build the mechanics as close to RAW as they can, I believe that it could be a really attractive venue for developing digital modules like Neverwinter Nights, and I feel like even small things like currency can go a long way to making the game feel much closer to the P&P experience.
The reason that they aren't using different currencies - at least at the moment - is because money is a physical item in the inventory, and right now there is no system for say, dragging 35 copper pieces onto a pile of 21 silver to create a pile of 23 silver and 5 copper, or for taking 3 silver and turning it into 30 copper.
In theory such a system could be designed, but I don't know if there are any plans for implementing such a system. They might not think it worth the design effort to implement.
Such a system also needs to take into account all sorts of potential issues with closing the inventory or the player in a multiplayer game suddenly being disconnected, which may or may not be added complexity compared on top of normal inventory management.
One thing I'd like to see is a way to easily separate currency from the rest of the inventory, like a coinpurse that automatically accepts all currency upon loot, but still treats currency as physical objects so you can move and split into stacks manually. Otherwise I don't see how multiple forms of currency would complicate the system overmuch. If you'd like to convert currency into higher-value forms like gold and platinum, there are banks for that.
