Nashville numbers are a system that allows musicians of varying degrees of literacy to communicate and work together efficiently. It's a simple concept that has evolved and been codified while still leaving room for individuality and improvement. I use it a lot when I have to make a chord chart in a hurry. A previous poster compared it to figured bass, and I totally agree. Its limitations as a notational system become evident when harmony gets more complex; minor keys can get a little weird as well.
Exactly. But having devised my own variant of it around age 9, I still think it's stupid to be forced to call it "Nashville." (And I love country music...)