The cookie use case is just one use case. Another PSL use case is helping browsers decide what part of the FQDN is most important for the user to notice. On firefox I see "ycombinator.com" in bold, with normal font weight on the news subdomain. On smaller screens the URL bar isn't left aligned, it adjusts the view to prioritize showing the relevant domain name. You don't want an attacker to control how the URL is displayed in that security context.