I think Bitbucket is pretty good, and has some advantages--most notably that I can have private repositories there without paying. Yes, this may identify me as a cheapskate, but, well.

If anything, I suspect this investment is to make Github more competitive with Bitbucket -- remember, BB's parent company is Atlassian, which has pretty deep enterprise experience they can leverage. You may like GH's issue system better than BB's free one, but a paying BB customer can integrate it seamlessly with Jira and other Atlassian tools.