Slimeballs are also green! It's like they're asking
to be used as some sort of currency.
Money also means we can build shops. It also means that people can charge for things, like use of turtle's armory, which means I won't feel as bad about using so many diamonds when I fall off the skyway like a n00b, lose my diamond armor and need to take a new set
It makes trading between players viable too. If you mine/collect too much of something this makes it possible to convert that surplus into something useful. If we adopt currency I'll be selling things out of my house in Bridgeport for sure.