Traffic or pagerank

I have long pondered this... why do advertisers rate your site by pagerank when they are assessing a site for suitability??
Well my conclusion is that advertisers (those who pay for paid posts) are actually planning to increase the likelihood of their own site popping up in search terms. Although I have good rank on the two blogs that I do paid posts for, neither of them receive huge amounts of traffic. So if the advertiser wants a high PR, rather than high traffic and high visibility for the post (though I am sure they object to it!!), then he must be looking to raise search engine visibility for specific anchor text primarily. Not all advertisers request specific anchor text though, so I guess any back-links with relevant text is the aim in those cases.
If you were advertising on a bill-board, you would look at the position of the board. Is it visible to lots of people, is it visible to my target audience? In the case of blogs, the advertiser is concerned with the back-link value rather than the passing trade, or so it seems...