ISPs or anyone could still insert malicious javascript into your site, or present download buttons or login features on your site. Visits to your site could be

ISPs or anyone could still insert malicious javascript into your site, or present download buttons or login features on your site. Visits to your site could be logged by IT departments, getting your users in trouble. Also I've actually heard of places where port 80 is actually blocked by default (but 443 isn't) and you need permission on a site by site basis to get it unblocked.