It's less pageviews and more reader engagement. You want to provide your salespeople solid evidence to convince the advertisers to pay the premium rate.
While perhaps the last part of the deal is measured in ad impressions, all the other facts and figures are important: time on site, pages per unique visitor, where the site's traffic is coming from, etc.
I agree with what you're saying about pageviews as a metric for reader engagement, but it still seems like in an ideal world if the advertisers had a solid empirical measure of ad effectiveness rather than just a count of the impressions, then pagination would have no impact on ad revenues.
While perhaps the last part of the deal is measured in ad impressions, all the other facts and figures are important: time on site, pages per unique visitor, where the site's traffic is coming from, etc.