When moving from svn to GitHub what you're actually doing is moving from a centralized svn system to a centralized git system.
The big difference is that in the second case you can keep working on your local repo without touching the central repo, at any time add new remotes to your local repo and pull and push from your peers.
If GitHub is down, you just keep working. If your svn server is down, you just pile your local work waiting for it to come back up, the tool will not help you in that case.
Moving from svn to git is a no brainer, even if you keep using it as if it were svn most of the time.
If Github would only host git repositories, you'd be right. But people use Github for the issue tracker, source browser, code review system. Those are just as centralized as the svn server. And in my opinion, they are at least as important as a source control server to get things done.
The big difference is that in the second case you can keep working on your local repo without touching the central repo, at any time add new remotes to your local repo and pull and push from your peers.
If GitHub is down, you just keep working. If your svn server is down, you just pile your local work waiting for it to come back up, the tool will not help you in that case.
Moving from svn to git is a no brainer, even if you keep using it as if it were svn most of the time.