A network like DoubleClick has sites such as Network Solutions that would hit the target dead on. Package a few targeted sites together, and you might be able to reach critical mass. Dealing with a network allows for single buys to cover a number of targeted sites in one fell swoop. Another advantage of dealing with a network is the depth of its ad sales force. While a tiny site may have an advertising sales rep, it's not cost effective for it to also have a sponsorship salesperson, a promotions salesperson, and a direct marketing salesperson.
Networks can bring that to the table for smaller sites.
As a result, media planners can work with experienced salespeople to bring customized advertising packages to smaller sites, something that normally only the largest portals and content sites would have available to them. Networks can also be good for providing a cheap way to test ads against different targets. Segmenting the network buy and determining response by individual site can help to show the response potential of various audiences. I don't think it's a good idea to stigmatize networks because of the way in which they make things easier for the buyer. I think that all potential media packages should be evaluated according to their individual merits when an agency is in the planning process.