This is less of a problem for pay-for-action type ads. If you send X number of people to your site with Y conversions, it doesn't really matter that 'bots don'

This is less of a problem for pay-for-action type ads. If you send X number of people to your site with Y conversions, it doesn't really matter that 'bots don't buy' - because the $Z spent on ads for Y conversions is the metric used. It's much more of a problem for impressions or for ads without directly measured value. It's one of the reasons Google makes so much money, the spend is generally optimized for the return, not for 'how many visitors are fake'. Though it's a problem.