I think this really applies to all online courses. It's great to take a marketing course, but to really become a marketer you're going to have to learn how to learn on your own. Would be great for courses like Udacity to explicitly build material into their curriculum that teaches you how to learn on your own after the course is over.
I'm currently working through my first Udacity course (computer graphics). The problem sets at the end of each lecture are open-ended, leading to the same problem-solving pattern that I use for self-education: Google, StackOverflow, Wikipedia, etc. I believe students who complete that course will be prepared for continued self-education, unless they skip the problem sets, but perhaps other courses aren't as well designed.
Very true. It takes projects to separate what's needed versus what's purely academic. Projects also force you to learn what isn't taught, and to open the hidden doors in the technologies that you're using.
Similar with your analogy - For a marketing position, I would rather hire an English major with 2 years brand management experience at P&G than an MBA who has never worked in Marketing.
I had read a lot of marketing books and watched videos. Then I tried making my first Google Ad. I lost hundreds of dollars. Actually doing stuff is very humbling because you realize how little you know.