Conversely, the protocol bit is relevant whenever your link is not domain- or protocol-relative, or when you can't be sure.

And you can't be sure. Here's why: if someone saves your page on their hard drive and you're using protocol-relative URLs, the implied "http" just became "file". (Which in fact may be a nice excuse for some clever hacks.)

Test it yourself! Save the following snippet to your computer and see how your browser resolves the URL:

  <!DOCTYPE html>
  <title>Testing 1 2 3</title>
  <p><a href="//google.com/">Linky</a></p>