For paid submission directories running any type of ad network competes with the listings in the directory. This I believe is unfair for paying customers.
chabbs, where ya been all this time? ;)
I envision directories and/or vortals as self-supporting entities. Most advertising should be from the companies listed within the directory, not outside of the directory. This type of advertising has many benefits and side effects too.
I just think with the proliferation of "Made for AdSense" sites, that my perceived value of those ads does not justify jeopardizing the integrity of the directory.
At the end of the day, they're still ads, and the site simply has higher credibility without ads, which i feel is especially important in the beginning, as here the credibility has to be earned. Once the majority of users are no longer new to the site, credibility will be based on experience and hence ads will be less disturbing.