For many of these sites they might as well not have you login at all and allow you to do whatever they let you do with OpenID, since after logging in they only get a unique identifier (not even an email address, am I correct?).
I see where you're getting at, but I think it's more of an issue of keeping your digital identity intact rather than fragmented over multiple accounts. I can authenticate at a single place, and that server can vouch for me on other sites ("Yeah, he's good to go. He logged in with me. It's him and not some other user."). With some sites you could get away with no login (see 4chan where everyone is anonymous), but for discussion sites that rely on reputation (stackoverflow, reddit, hackernews, etc.) that's not really ideal.
Security-wise, you have the added advantage of not having to manage multiple passwords. You could use a single password for all your sites, but then your password exposure would be too high, since a breach in any of those N websites could potentially capture that one password of yours. With OpenID, those sites wouldn't even be getting my password.
But then a breach in your OpenID account would mean access to every other site right?
However I do agree it would make it a little more secure than using one password at every site. However for smaller sites that I care less about, I generally use a special password anyways, and it seems it's really smaller, less ambitious sites are the ones that will adopt OpenID anyways. I don't think Yahoo, Google, or Facebook will start taking OpenID logins any time soon right?
