I guess that's the disconnect, because I do consider my email address secret -- since it is 1/2 of the information required to access my Google account, I take great measures to make sure that outside parties never see it (as best I can). If I can prevent any site from ever knowing that address, then my chances of being targeted (phishing, brute-force, whatever) are drastically reduced.

So, if you only use +tag for your own personal organizational purposes, then have at it! But if your goal is to conceal your account ID with Google in the interest of personal security, then you really need a better angle.