Now, there is something to be said about some signature models which have next to NO difference from the production models. I ADORE Biffy Clyro, but I, for the life of me, can't find anything especially appealing to me about James Johnston's Jazz, save for maybe the matching headstock. I don't have any particular desire to acquire that bass because it's not SPECIAL. I could grab a MexiJazz from Guitar Center used for $300, and that'd be the end of it. I could get the Biffy Clyro vibe (Though my Precision gets that vibe just fine.)
I can't see any difference between buying a James Johnston Jazz, and just buying a Jazz because you like James Johnston.