It was a mistake in the beginning too. If you want to serve different content to different people, you use two different URLs. There's never a valid case for doing it while hiding it from the user.
No, that misses the whole point of content negotiation (for language, et al). In the age of search engines though, different urls are often perferable.
> No, that misses the whole point of content negotiation (for language, et al).
There's been some pushing for browsers to automatically decide what language you want to view a webpage in, and that is even more insane than the User-Agent header is. The correct solution for content that exists in multiple languages is to make each language available at its own URL. Just like I already said. Why in the world would you want to lose the option to select the language you want?