Neil Harris wrote:
A possible point in favour: using rel="alternate" together with the hreflang attribute in <link> elements (as per http://www.w3.org/TR/REC-html40/struct/links.html#h-12.3, section 12.3.3) might help search engines make smarter decisions about indexing Wikipedia content, thus increasing the relevance and availability of Wikipedia content in Web searches made by the general public.
That's an interesting statement. What evidence do you have to support it? Can you name any search engines that actually look at this information?