People who would go to non-smoking bars but would not go to smoking ones. Given that bars are all smoking, they do not go to bars. Smokers who are part of the bar scene won't make friends of these people there because these people aren't there. Bartenders are unaware of these people since they aren't at the bar. They are an unknown audience that nobody knows how to estimate.
However the increase in attendance after smoking bans are put in place suggests that this group of people should not be discounted.
Cheers,
Ben