I wouldn't buy a bass solely because it is a "signature model," but I would buy a signature model bass if it sounded good, played well and looked good. Back in 1972 I bought a Rickenbacker 4001 solely because Chris Squire played one, but I did it for the sound, and there was no "Chris Squire signature model" then. If there had been and they weren't charging too much extra for it, I may have bought it. I love the old style Fender P bass -- single bar pickup, small headstock -- but I wouldn't buy the Sting model just because it has his name as a 12th fret marker. None of the current crop of other Fender signatures interest me either, although I did rather like the Roscoe Beck models.
However, I do have a Hofner Icon and I'll have to admit that if McCartney hadn't played one, I probably never would have looked at Hofner in the first place. But I was not going to pay an additional $1,500 for the German 500/1 Beatle bass. The Icon looks close enough and sounds very much like the German model, and that's enough for me.
I just bought a Rickenbacker 4001C64 in Fireglo, which is patterned to replicate McCartney's Rick before the psychedelic paint job (and subsequent stripping/sanding). OK -- some of the reason is because McCartney played one, but more is because I like the Rick sound, wanted the horseshoe bridge PUP and prefer the unbound dot-marker neck over the bound triangle-marker neck that I had on the 4001.
I absolutely can't see coughing up $8,000 for something like the Warwick Adam Clayton Reverso or even $1,500 their over the top Bootsy model.