I have a few 'signatures' but mainly because they are great basses that you can't buy in other guises.
Epiphone Jack Casady, as already mentioned it's the sensible way to own a Gibson Les Paul bass.
A Yamaha Attitude, a Precision but so much more.
A Yamaha BBPH, the sensible way to own a BB1200s. Actually more of an amalgamation of a 1200s and a 734a.
An Eastwood Hooky Pro 6, the sensible way to own a Shergold Marathon 6.
It helps that I'm a fan of Hooky, and although I totally appreciate Casady and Sheehan's music I'm not exactly fanboi enough to buy a bass just for the sake of it. All four of these basses are fantastic instruments in their own right and unavailable to buy as a non signature model or anything close by any other manufacturer.
Ironically very few people outside of the bass world would link these basses to their signature players, yet pick up any generic violin bass by any manufacturer and people will say McCartney.
Play what makes you happy regardless of the reasons.