The value of content is subjective. The value of Steem is based on the value of the people behind it, the Steem community. Stealth marketing is already happening but you're absolutely right.

If you look at the makeup Vlog it could be stealth marketing right there and people would never know it. I mean isn't it great for the companies who sell makeup? And nothing stops these companies or any other companies from giving sponsorship rewards in their own tokens.