What happens if an agency gets in deep with one of the common trusted authorities shipped with every browser, or is an authority, or just hacked their root keys, or bought access like they did with RSA? It seems like they could man in the middle all day and the only difference would be the cert issuer, which means it would be invisible if used in a limited fashion.
Well, I don't really trust random certs even when they're signed by a respected CA -- but I still prefer using HTTPS. Even if the cert is fraudulent, HTTPS is still encrypting stuff and will protect me from other random attackers.
Security is never a binary secure/insecure proposition. There are shades of gray. The key is to use what security you can, but never think "I'm secure now".
As an old mentor once told me: the moment that you think you're secure is the moment that you're at the greatest risk, but you should still lock your door.
