This is also an accessibility issue. When there is a primary language for a document (be it HTML, RSS, or anything else), it's advisable to indicate that for the benefit of screen readers, search engines, aggregators, etc.
OK, so it's an easy one-line addition to each of several files. But most people publishing with WP probably have never even read the RSS spec, probably don't even know that this tag exists. They aren't going to add it without a massive education campaign like the Web Standards Project or Mozilla evangelism projects. Even then, it'll only be those comfortable with hacking PHP and/or XML. For many, the whole purpose of using a publishing tool like WordPress is so they don't have to worry about coding!
Alternatively, a small, easy change can be made in the authoring tool that will make their feeds easier to index and thus easier to find. IMO it doesn't even need a UI, as long as the language is stored somewhere. It would be one of those sensible defaults.
As for "bad or false" metadata -- I don't think anyone here is advocating that. If the feed contains multiple languages, it seems to me that would be the case for leaving out the tag.
The only reason I can see that this feature would not be easy to add is if WordPress does not know what language it is being used in. I have been assuming that, with the localization push, WP would have this stored somewhere. If not, please correct me.