Linux doesn't need marketing Linux is a growing product and it's maturing at a rapid pace. Big companies like Motorola, Sony, etc are using Linux as an OS of choice because of its power, extensibility and awesomeness.
While I certainly agree that any marketing objective requires measurable metrics to judge the performance / quality, it's not dependent purely on sales volume or variety. The best example for this case is Apple and its marketing of the iPhone.
Everybody knows what an iPhone and the product itself is awesome. Then it doesn't make sense for Apple to market iPhone does it? It's just money going down the drain, when they can use that money to boost their earnings.
Apple continues its iPhone marketing campaign to pass on uses and experiences that you can have with the iPhone. Furthermore, the hand actions need to be shown "somewhere" for people to realize how intuitive it is. They are using the iPhone campaign to promote that as well.
Hence, the marketing is not all about sales and volume, but also to get a certain message across.
Linux could certainly do well with some marketing campaign promoting its ease of use (to dispel the geekiness and how hard it is to setup factor) in the generic desktop computers and also its amazing productivity and security features.