Good point on ReportURI having better abilities to detect false reports. It's not mentioned (at least, not on the front page; I've not delved), but definitely

Good point on ReportURI having better abilities to detect false reports. It's not mentioned (at least, not on the front page; I've not delved), but definitely their larger dataset will make it much simpler to blacklist IPs suspected of sending faked reports / spotting patterns to remove false data. I believe the benefit of having users' browsers report this over a crawler is for scenarios where pages attempt to display your content from another site (e.g. in an iframe behind an overlay for a click-jacking attack). You'd never know to monitor that URL / wouldn't know that the site was hosting your content from any of your metrics; but the users browsers would report it. In terms of "why report it if they know to block it anyway", I believe the idea is to improve security for others; i.e. we're no longer relying on users having the latest browser to be protected; so long as one user had a browser good enough to spot the issue, we can be made aware that there's a risk out there.