One of the effects of having a mainstream media that has long been supported by advertising is that advertising itself serves to legitimate discourse around it as being serious and professional. That someone would pay to place an ad near some writing makes that writing appear valuable in a way that the quality of the writing alone can't necessarily do, especially if it can't draw cultural capital from any other source -- from the notoriety of the writer or the publication or what have you. (The health and competence of national magazines' editorial staff can often be gauged by the quality and quantity of its ads.) I used to see the formation of artist/writer communities, that serve at once as producer and audience and support system for the production of new culture, as a potential counter to that sad fact, and I still believe that it's inherent for humans in general to trust their social networks to confer legitimacy and relevance. (That's why the MySpace marketer infilatrators so disturb me; they are coopting the social network and poisoning it at the root, undermining its important legitimation function.) But for better or for worse, the presence of ads also confers it, and the more expensive the ads seem and the more reputable the companies that they stem from, the more prestige for the media to which they are attached.
Just as home recording and technological advances brought a flood of indistinguishable and overwhelming music, so has blogging unleashed an avalanche of unedited prose. If blogging has made it easier for amateur writers to attempt to make their voices heard independent of the legitimation confered by ads in our culture, it has also made it easier for them to flood and drown out each other, yielding no net gain, and making the filter afforded by advertising legitimation, the filter of media buyers seeing a certain writing as audience-worthy in advance, all the more necessary.