I guess it depends a bit.
- I bought a Jack Casady once (and sold it again), when I was playing bass for a short time, but I had to look up who Jack was:)
- I bought the Billy-Bo, because of the shape. I love Billy Gibbons, I love Bo Diddley. Both don't play bass, and still they have a sig sound.
- I bought my Thunderbird because of Entwistle, but of course that was no sig.
- I'm always GASsing on a lot of basses and the Mikey Way Mustang is one of them. And I dislike MCR.
- If I had the money to spend I would love to have a Pino sig. Great bass!! And if I played guitar I would buy the G-Love Gretsch immediately.
- The Manchester Blue Noel Gallagher (so not the Union Jack) was quite a seller, because it had better pickups than most other Epi's. Most people cleaned the signature on the pickguard of that guitar:)
So a lot of very different reasons for buying or not buying sig guitars and basses.