I think this Salon story gets it about right. When campaigns contact me about advertising and ask for information to justify the expense I'm usually somewhat discouraging - telling them to not advertise or to buy up ads on a lot of cheaper sites. Or, at least I try to lower their expectations. Sure, the advertising department here loves the money but the activist side doesn't want them wasting it.
That isn't to say candidates shouldn't buy blogads, but they shouldn't buy them expecting to get quick return. Ads should be seen as part of a wider longterm internet outreach strategy. One should see bloggers and blog readers as another contituency which can be appealed and pandered to. Candidates and campaigns which spend time and effort reaching out to that constituency may be able to reap some greater rewards over the course of the campaign. Fundraising for Kerry is going to stop cold at the convention, and people are going to be looking to see where else they can donate to. Having name recognition at that time could be a big help.
But, if candidates are going to judge the ads as they'd probably judge most directly mail money pleas (We spent $X to mail the things out and got $Y dollars in return in the pre-paid envelopes) without doing any further internet outreach then they're probably wasting their money.
