difference between track.js and track.asp

Though I haven't looked into the coding of either very hard, i notice that the .js logs the screen information and then just calls up the asp page and runs as normal with the bonus of adding the screen information. Bearing in mind your note "Don't put more than one tracking method per page", why aren't the default instructions: put track.js on every page - because it does everything the asp tracker does, plus more?!! Is it just because if a site user doesn't have javascript enabled it won't log?

Thanks for any reply.