I think these sites (eg Perez Hilton) still rely on resellers like Blog Ads because they need to be aggregated into packages of page views larger than they can generate on their own. EG Perez generates 1MM page views a month and Snickers wants to buy 50MM impressions, so Perez becomes part of a collective that can generate 50MM across all the different blogs.

But then when Perez reaches 50MM himself the rather large part of his markup going to Blogads stops making sense.