On my projects...
... I always make a tarball available, but bandwidth isn't the reason: I do it because I figure there are still a lot of potential users out there who haven't used hg and who don't want to have to figure it out (as simple as it is) before they can look at my project. That's also why I provide a zip archive if there's a chance that the project can be useful on Windows. Even though tar versions are available for Windows, why put up that hurdle?
As far as bandwidth goes, I have yet to download a project where the difference between the size of source control and source alone is noticeable. But then, I've been on 1.5mbps+ for the last five years, and now I have a 16mbps connection. YMMV.