Why use protocol-relative URLs at all?

Solution 1:

As of December 2014, Paul Irish's blog on protocol-relative URLs says:

2014.12.17: Now that SSL is encouraged for everyone and doesn’t have performance concerns, this technique is now an anti-pattern. If the asset you need is available on SSL, then always use the https:// asset.

Unless you have specific performance concerns (such as the slow mobile network mentioned in Zakjan's answer) you should use https:// to protect your users.

Solution 2:

Because of performance. Establishing of HTTPS connection takes much longer time than HTTP, TLS handshake adds latency delay up to 2 RTTs. You can notice it on mobile networks. So it is better not to use HTTPS asset URLs, if you don't need it.

Solution 3:

There are a number of potential reasons, though they're all not particularly crucial:

• How about the next time every business with an agenda pushes a new protocol? Are we going to have to swap out thousands of strings again then? No thanks.
• HTTPS is slower than HTTP of same version
• If any of the notes listed at caniuse.com for HTTP/2 are a problem
• Conceptually, if the server enforces the protocol, there is no reason to be specific about it in the first place. Agnosticism is what it is. It's covering all your bases.

Solution 4:

One thing to note, if you are using CSP's upgrade-insecure-requests, you can safely use protocol-agnostic URLs (//example.com).