I'm the cat who's never paid more than $400 for a bass, which means I read threads like this to see what I'm missing out on. I have some really wonderful basses that are a joy to play, but I'm always curious about what a "really good bass" could bring out, or encourage, in my playing.
What I love about playing different basses is that each one seems to have a thing that it wants to do; a place where it really shines. Sometimes, I can't get a certain groove right on one bass, so I reach for the one that might be a step ahead of me. I could see a very well-made bass offering up more room to discover what you can do. If I only stuck to one bass, I don't know that I would push my boundaries as much. I'm not sure if that's an argument for one great bass or not, but I sure would like to play around with a few "great" basses at some point! Maybe not when they are economically out of reach, though.