That tidy solution only works if the fraud rate on the ads you place is less than or equal to the industry presumed/priced-in average. It seems that there are more than a few ad networks around these days that specialize in non-human traffic. As such, advertiser realized fraud rates can vary greatly from this average.

My company right now won't purchase RTB ads that display outside of walled gardens so that we can 1. manually verify that the ads are showing. 2. Minmize the number of bots seeing the ads (presumably it's hard for bots to infiltrate Facebook or the WSJ than Buzzfeed).