Okkk, now we're getting into two different things, which are, however, related:
1) The rider's impact and value to the team when measured by racing wins or in-race performance in service of the attainment of some sporting goal.
2) The rider's impact and value to the team in furthering the marketing goals of the organization and its sponsors.
The second can certainly be enhanced by someone at the top of their game in the first, but - as others have pointed out - even a basically win-less Cav still delivers marketing value because he continues to generate media impressions that have not yet become so clearly detrimental to the images of the team and its sponsors that management has to step-in and muzzle him.
Without going into great detail, just b/c Greipel is German and his home market is hostile or tepid to cycling doesn't mean that his sporting performances this year aren't contributing to the furtherance of #2, above. It all depends on what the team and sponsors' marketing and business goals are, and how Greipel's image and performances are used in the campaign to realize them. Whilst Greipel is German, just because German cycling is in the doldrums, and the media predisposed to negative commentary on cycling, doesn't mean that Greipel's effectiveness as a brand ambassador is marginalized. He might be German, but cycling is a global sport and the impressions he generates are in media scattered around the world.
One of the few hypothetical scenarios I could think of where Greipel's winning would be bad for business would be if his victories came against (head-to-head, despite being teammates) and at the expense of a Cavendish who: was exclusively using and endorsing a particular product that Greipel did not even have access to. For example, if Scott created a "Mark Cavendish Blowhard" carbon team replica bike and it was promoted so heavily as being the embodiment of the essence of Cav - and no one else (least of all, Greipel) - and that in advertisements, team statements, company press releases and the spoon-feeding-of-copy-to-journalists, it was unequivocally described as the only bike capable of being ridden to victory in "6 or more stages of the Tour de France...and in the German equivalent of the parking-lot criterium (or whatever was the race in which 90% of the German market would ride). And it was the ONLY road bike Scott sold in Germany, and Greipel rode some other Scott bike that wasn't available to the German public - that wasn't even in the catalog (!!!) - and which was not promoted as a viable sporting or commercial option.
Far-fetched, but then each time Greipel won - anywhere in the world - and Cavendish didn't win, and the reports filtered back to the German press that Greipel won aboard his unavailable, afterthought-of-a-bike, which Scott dealers in Germany couldn't sell, while Cav's dream machine remained win-less, the Manx Missile's bike would be extremely unattractive to the buying public in Germany.

(eventually, of course, Scott would have to launch a new ad that revealed the big secret - that Greipel had been riding a stealth version of the same bike all along - haha
lol ... anyway, I probably didn't need to create that silly example - you knew what I meant: a rider's PR value is not limited to his country of origin, unless that's the way in which the team and the team's sponsors decide to use him, but he fails to support the combined sporting and marketing efforts therein, (or the team and sponsor's fail to unlock his full promotional potential w/in the home market).