I can't afford to give full support to all locales (translation costs are ridiculous), so detecting and supporting the user's locale seems to be pointless unless they happen to be using one of the limited set I can give full support to (en-CA and fr-CA in my case). It seems like a far better idea to allow the user of my website to choose one of the supported locales, and see everything in the same consistent format than to have a few input widgets on each page decide to configure themselves based on the locale the browser was compiled with.
Edit: also, consider testing. Am I supposed to download a bunch of variants of Firefox to test my locale support?