I don't think we should discourage people from setting up systems that have this kind of accessibility, even if it's not as "pure" from the perspective of duplicated content.
(The guidelines say it's preferred for everything to conform directly, but they use "artistic integrity" as an example of a situation where a conforming alternate version could be used. I'd say that applies here.)
So we install smoke detectors and put expiration dates on things to detect danger without smell.
You can't make a text description of a guitar solo that's "equal", much less not "separate"; I don't buy that people really think that we therefore shouldn't have music.
Everyone I've heard from thinks a separate site is better than a massive alt though. Many screen readers handle them extremely poorly, and they can't be annotated with html tags to indicate semantics (link, list, etc).