I guess clicks are the primary metric Google has to evaluate the quality of the ad; if the ad is rarely clicked they can assume it's not relevant to the keyword

I guess clicks are the primary metric Google has to evaluate the quality of the ad; if the ad is rarely clicked they can assume it's not relevant to the keyword. The other thing is that impressions have a non-zero (albeit generally low) value for the advertiser, but Google doesn't charge for them. So they have an incentive to disable ads with that profile.