You can't say that micropayments doesn't work when ads ARE micropayments. They're just micropayments with a particular set of properties.
The only question remaining is which of those properties are necessary. Is it actually necessary that your nickel causes an ad to be displayed, or is it just easier and the way we've organized things so far?
So, no, it is by no means a micropayment. And even if I were charged for overage, and I were over my monthly limit, I wouldn't be charged at a rate of $1 for 20 MB.