Quality content is quality content. I don't care who posts it. If the content helps someone, answers questions, provokes thought, provides insight, adds clarity and/or otherwise enhances the forum discussion, then it shouldn't make a difference.
OTOH, if a source shows a pattern of frequently posting content that doesn't really add to the discussion then I would take notice and pursue action to stop it.
In my experience, most sources are singularly concerned with sales and therefore incapable of the former. So I don't have a problem with the general discouragement of sources "posting outside their thread". It saves me a lot of time cleaning up the spam.
However, I would advise that everyone judge the content on its own merit before they consider disrupting or derailing thread discussions.