I'd say it's quite an argument. If 90% of people do it, it means that there is a 90% a prior probability that your other candidate has done it as well. So there

I'd say it's quite an argument. If 90% of people do it, it means that there is a 90% a prior probability that your other candidate has done it as well. So there is a 90% probability that you are basing your decision entirely in a non-factor. Surely there are more meaningful differences* between candidates to take into account that one that has 90% probability of not even being there at all. To each their own, though. *I know you said "otherwise identical", but that's not a very realistic situation and if it did happen, it would justify choosing by just any irrelevant difference, like one candidate having one day of work experience more than the other, so I don't think that says much about the importance of a criterion.