It's not quite the same as having native hosting of Subversion and Mercurial repositories. With a bridge and plug-in there might be some edge cases where things don't work as expected. I wouldn't risk my source code in this way.

By that logic github allows you to use CVS, bzr, darcs, monotone, TFS, whatever other VCS exists that has a git bridge of some sort. You miss out on any benefits of using that VCS over git and you introduce a huge performance sink.

Github's subversion bridge in particular is pretty second-rate - it's really only useful for IDE integrations which don't support git, and even then it's of questionable value. It falls over or provides incorrect data for almost any SVN request that falls outside the standard checkout-update-commit workflow.