I find this too prohibitive. Even with the exception this would make me think twice about a promo post and maybe even refrain me from posting at all. For non-except services it is even worse. It may lead to spam posts or users trying to categorize contributions into useful and not useful posts.
Self hosting does not end for everyone with free services. Some of us are happy to pay for services provided by others and I would really like to read about these here as well. I know this is not the intention of the rule, but it will be its result.