Mark points us to this great set of stats.
Basically, all of the clicks for all the ads online come from only 16% of the surfers, and most of them come from just 4% of all internet users.
So, if you optimize your ads for clicks, it means you're ignoring a huge population.
If your business is built around the kind of person who clicks, you win. If it isn't, you either need to not buy ads online or buy ads optimized for attention and familiarity, not clicks.
Imagine that only left-handed people clicked on ads (it's about the same percent). What are you going to do if you make a product for the right-handed portion of the population?
It's okay to make an ad that isn't easy to measure. If it works, that's enough.