In reply to Danny Brown.

? Comments didn't get much love for a long long time and there is probably much to improve with the UX. At least moving the important part of the comment to be higher is now done (if you are bored - my rant on this subject from 3 years ago - http://eng.marksw.com/2013/01/01/almost-all-wordpress-themes-suck-at-comment-form-design/)

The problem with registered user commenting without being logged-in is that WordPress core do not know to associate the comment with the user and depending on your anti-spam policy the comment might need to be moderated.

my solution (plugin here - http://shop.marksw.com/downloads/prevent-comment-impersonation-plugin/) is to actually force user login when the email address in the comment matches a registered user's email. But this of course only half way solution to the problem you raise and there still needs to be a way for a logged in user to comment as someone else without logging out first.