Welcome to WebmasterWorld clearvision!
My view may be biased as we sell impression based advertising (CPM) on our own highly targeted sites, but given that here is our view on advantages for both CPM and CPC.
1. You can piggyback on the authority and credibilty of that site. Most sites do have stricter selection guidelines for their advertisers than general PPC search engines. That translates to an increase in trust to people who see your ad. To me this is the prime advantage.
2. People who visit those sites regulalry may see your ad more, and as we all know it takes a few impressions for your ad to register.
3. With PPC search engines, you are competing against many others.. ranging from affiliate sites to people trying to set up their own directories and second-party marketing services, plus some sites that are much more general than your specific product. Single sites can target you ad much better in many cases.
4. Becuase single sites are usually smaller and very targeted, they know their specific market better and sometimes this translates to more flexibility for anything from size and weight of banners, copy wording, links, timings, packages including insertions in their newletters, opportunities to be included in advertorial, frequencies and which pages the ad appears on.
5. Single targeted sites are also better for b-b rather than consumer sites. I think PPC sometimes is better for consumer items.
Of course I am sure there are advantages of advertising in PPC engines too, but I will leave that to PPC search engine reps!