With the exception of broad match, advertisers choose the keywords for which their ads appear. Unless this idea was only in regards to broad match keywords, I don't see how such a system could be implemented to the advertisers satisfaction.
"You asked for your ads to be shown for [green fuzzy widgets], but this one guy was never clicking an ad when he searched for [blue shiny widgets], so I showed him your ad and he clicked! You do sell blue shiny widgets too, right? Oh, you don't? Well, at least I got him to click an ad!"