I've found the hg-git bridge fairly usable going the other direction (exposing some stuff on Github that was originally built using Mercurial for source control). Basically, you "just" use mercurial, create a bookmark linking "master" to the "default" when you're ready to push to a git repo, and push. Pull from git works similarly to the way it does when you pull from a remote repo. You don't end up with .git + .hg files in this scenario, you just have mercurial ones on your local box, unless you end up migrating over to git entirely, in which case you'd probably do git clone in a new directory.
There's a similar project for git users that want to push/pull from Mercurial repositories, but I don't have direct experience with it. See https://github.com/offbytwo/git-hg
Other than the potential of accidentally checking in a bunch of the hidden folders from your "other" source control system, I don't see any real problem with doing hg init in a git repo; it just seems less clunky to me to use a bridge, and explicitly choose when to push to the other dvcs system.