Is the dns-prefetch only useful for things that are not already linked/included on the page? (such as redirect chains... t.co links might DNS prefetch the domain of the end of the chain)

Or is the benefit that you can start the DNS resolution whilst receiving the <head>, for the JavaScript resources only included near the </body>? (being a very slight performance benefit)

Edit: And I suddenly thought... for SSL pages would dns-prefetching leak some information to an observer about the composition pf content or the linked resources on a page by grouping such DNS requests into a short burst at the same time as the SSL request?