I think I get it now. There seems to be a confusion about what specialization means. It means specializing in the service you provide, not in the customers you provide it to. Market segmentation is only a tool to identify how to specialize your service. But no sane company would refuse to deliver to a paying customer simply because he doesn't fit into their target audience.

And there is a difference between computer programming and basic marketing. The former is a specific skill with a smaller area of application while the latter is a very general skill, and what is more one that stems from a basic human trait, namely the formation of relationships. Of course, not everybody needs specific marketing knowledge as taught in business administration.

Finally, I'm not arguing against working for a single employer in general. Quite the contrary. When you're relatively new to your field of work you almost certainly lack the experience to be a successful entrepreneur and should first learn the trade under the relative security of employment. What I am arguing is, that if a huge number of people do not gain the confidence from experience to form their own idea of the service they want to provide and market it to a relevant audience something seems to be wrong, because taking responsibility for your life and forming relationships is an essential part of growing up.