It used to be (>10 years ago) that it wasn't worth it to get factory "premium" sound systems because a well installed aftermarket one using quality products usually sounded much better and might offer more features as well for the same cost. These days, however, the electronics are so highly integrated in new vehicles that it takes a lot of work (and cost) to replace the features and functionality of the premium systems with aftermarket alternatives. For instance, if you left off DVD, and later wanted install an equally functioning system, you'd have to contend with sending video to the Nav screen (not that hard), dolby digital processing, wireless transmission, and proper selection and control through the factory Nav unit and steering wheel controls. No easy feat, and likely a lot more expensive than the Bose system. It might sound better, but it just isn't as easy a proposition as it once was.
As an aside, Mercedes has used Bose in some past vehicles, and in particular the late 90's S-class models have wonderfully executed Bose designs. So the company has the knowledge when they are contracted to apply it.
