Isn't one of the main benefits and best things about the internet is anonymity? Obviously FB and Twitter have information about my real identity, but why would Instapaper, or Reddit? Having a global login takes away that anonymity.
There's nothing stopping you from using an anonymous Twitter account to login if the site supports Twitter authentication.
Alternatively, MyOpenID and some other OpenID providers allow you to choose what pieces of data to present to the requesting site when asking you for permission to authenticate with them.
Facebook is another story, as they want every profile to correspond to a real-life human and only present an allow/deny choice for permissions to the requesting site instead of finer-grained access control.
The ability to be anonymous is an important one and should always remain available, but in general day to day usage anonymity is only used by people who don't want to take responsibility for their own actions (or words).
I see no problem with tying my offline identity to my online one.
Besides, if you wanted to disconnect the identities you just create multiple OpenIds for each identity you want to act out.
FTA: If all I want to do is leave a comment on a blog -- like, say, this one -- then one of the weaker forms of identity will surely do. If I'm starting a new bank account, or setting up a profile on a dating website, then maybe a stronger credential from my virtual wallet is necessary.
