I think this is a really good question.
My best guess is that what is old might become new again. That is, it used to be en vogue for ad networks to model themselves after rep firms, with exclusive representation deals inked with publishers. When DoubleClick was in the ad sales business, this was its model. Some smaller firms have latched on to this model and kept with it over the years, but most networks have moved away from it.
via An ad model poised for a comeback – iMediaConnection.com.
