I found this, from an article from Bass Player magazine.
BP: Are you still playing your '72 Precision? (one of his older basses)
AC: "I am. It's got a bit less varnish than it once had, but it's still around. I see photographs of it from different tours, and I can see the varnish gradually wearing off. It's a really light instrument, which is fantastic, because it's got this nice brightness without losing any bottom end. I'm always changing something on it, but it's still pretty much the same instrument I've always played. I did put a Jazz neck on it very early on; I find the Jazz neck suits my left hand better. The Precision is a painful, physical thing to do battle with. The Jazz is a bit more ladylike."
I think it's the case, until they started really making money, he would get basses that were in a rather rundown condition, and sort just fix up an older, used one.
Perhaps those older basses have their own sorta "character" which is why he still played them. If a bass is a custom, rare to find color, but has some dings, you can fix it up, make it look new or leave it the way it is. Maybe for him, as long as the bass sounds great, the look isn't as important.
He does have some brand spanking new basses, but I like how he'll still play the older ones, the "aged" ones.
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it is similar to the green on Edge's car 
