I'm not sure about the value proposition of marketing a mainstream VW vehicle which can only accommodate 2 rear passengers. Assuming that the Mercedes CLS and VW CC will appeal to different socio-economic segments based on price, I can also assume that CLS customers are more likely to be able to afford a second vehicle which can carry 5+ passengers when the need arises. The CC will have an uphill battle to achieve volume sales in the family driver segment.
But how often do people use their rear-center seat? I've never used mine in the Jetta. I think I used it once in the Legacy. I would have no problem buying a four-seater instead of a five-seater as a family vehicle.
Besides, if an argument can be made for the Accord and Altima coupes, an argument certainly can be made for this. They use family car underpinnings, but sacrifice a big chunk of utility for style. The Passat CC arguably gains as much or more style as a coupe would gain, with a much smaller sacrifice in utility.