If clearing the browser cache/cookies makes the browser forget about STS for each domain, then MITM attacker gets a lot more chances to intercept and attack. I don't have stats on how often average users clear their browsers but it is a fairly common troubleshooting step so most people are aware of it.

If clearing the browser cache/cookies does not make the browser forget about STS for each domain, then we got another way to maintain http://samy.pl/evercookie/