Need to track this study down because it seems from the coverage to be a bit wanting. Like much interactive agency-driven search research, Atlas approaches search from a banner-buyers perspective:
Atlas studied 30 search ad campaigns reaching 120,000 users on Google, Yahoo! and Microsoft. It found that nearly half of clicks on ads came from people who had already visited the advertiser's Web site. (AdWeek: Are Search Ads a Waste of Money?)
So, the other half haven't already visited the site. Even if you attribute no value to a repeat visitor via search, I bet the effective CPM on the clicks of first time visitors still looks better than the average banner campaign.
And how many marketers attribute no value to a repeat visitor. Does this sentence sound right? "It found that nearly half the visits to the mall came from people who had already visited the advertiser's store". Or, "half the recipients of the catalog had already ordered from the advertiser's catalog."