It may not be obvious to every golfer but I think most will agree the after market shaft is better than the one installed on mass produced drivers. When you buy a shaft you want it's characteristics to be what's specified in terms of weight, flex, kick point and torque. If the OEM shafts installed on drivers have looser tolerances then results can vary. I've read numerous reviews where bundled driver shafts didn't perform close to their after market versions. Will an after market shaft improve my score as a 20 handicap, not likely, which is why I don't spend $400 for an after market shaft.
I know some people justify the price of a driver ($400) because they believe the installed shaft is the same as the aftermarket version they see selling for $400+ online and in golf stores and decide that the new driver is a better value and investment than buying a new after market shaft for their current driver. In my mind that's not marketing, that's mis-representation. They should append an "SE" or "L" on the bundled shafts to indicate there are differences between them and what you'd buy as a stand-alone shaft.
Then you have guys like Robin Arthur (who put Grafalloy, AND the graphite shaft on the map) who come along and say "hey, y'all are getting screwed if you think that $400 shaft is worth $400!"