I think it was Hank Williams (blog: "Why Does Everything Suck") that pointed out that advertisers can not afford to buy ads on sites using this kind of model. The customers that micro-pay out of seeing the ads are exactly the small minority that the advertisers were hoping to hit in their blanket campaign. So the remaining traffic is worthless to advertisers- so better just to run it as a pure subscription site (which also has not worked well).