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.