Imagine for a moment that I wrote an article praising Longhorn in PC Magazine. Wouldn't you want to know that I'm a "professional commentator" on the topic and that I'm seen as biased on the topic?
But back to Scoble's PC Magazine article aspirations. Personally, I would find nothing wrong with Scoble writing an article praising Longhorn, as long as it is based in facts (as all good advocacy should be). What does McFarlane trash Internet Explorer (and, by extension, Microsoft) so thoroughly? Because IE does not conform to web standards in important areas like CSS, making life harder for web developers everywhere. Because it has security problems, partially because of this lack of conformity (MIME types are an excellent example of this). These are facts. In this case, Scoble's article would be relatively harder to write, since Longhorn has not yet been released, and many of the facts (not just concerning Longhorn, but its competitors, as well) may change.
Scoble brings up complaints by Dave Winer about an obviously biased Guardian article about the "conflict" between RSS and Atom. I feel that issue is different. Unlike The Guardian, InformIT does not have an "Opinion" or "Editorial" section. Indeed, they are not really a newspaper. Instead, they only have a loose collection of articles, most of which are written by advocates. Winer notes that his main problem with the Guardian piece is "It's an op-ed piece that's not labeled as such." While McFarlane's article is similarly not labeled, it does not appear in an unbiased publication, but on one where all of the articles are written by people who are biased.