Theres one area of your site that if you include it, users have no choice but to engage, if they wish to continue with the task theyre trying to complete.

That would be the CAPTCHA, other wise known as that annoying, often times barely legible word you have to recreate in a box, so that the site knows youre human. So if the user is already engaging with this, why not make it an ad? That appears to be Microsofts mentality, as it has proposed exactly that with a patent application. The concept is simple.