A note about the searcher and (b). There are a finite number of relevant quality results for every query. It is not the total number of results. It is much lower. In many cases there can be very few (10-20). Missing one or two relevant pages seriously downgrades the results. Does Joe know? Individually, probably not. In the aggregate, I think so. And of course there is the here today gone tomorrow issue with searches. Users do notice this.