Distributed workflow is still plenty useful, but the master copy being hosted by GitHub doesn't really affect it all that much. The real benefit was the ability to clone a repo and just start working on it locally - with commit history, branches etc - and then tidying it up and merging it back upstream when possible and convenient.

Are people forgetting what life was like back in SVN days, when not having a connection to the server meant not being able to do many operations?