Even with header bidding, true cpm can be a useful metric for measuring and evaluating different partners. Despite the fact that demand partners all now bid on an impression simultaneously, impressions can go unfilled by partners that win the auction. Ad partners may win the auction but users may navigate away from the page before the ad has time to load, leaving the impression unfilled. This kind of discrepancy can only be accounted for when using the rcpm and will give you a lower level of revenue generated from that partner than the traditional CPM which can hide this discrepancy.