[Disclaimer: I work for Twilio.]

It's actually not future-proofing, in that today we have different version of the API: 2008-08-01 and 2010-04-01. Some sort of versioning is necessary so that we can improve our API without breaking current clients or forcing all of our users to update their code. And while the goal of a completely discoverable API is laudable, I've never seen it work well in practice. We have a real service with real users, and we need something that works well and is simple to consume.

And I don't see how date-based versioning is any better than version numbers from a HATEOS perspective, since it's not really any more discoverable. There is however an argument to be made that version numbers are simpler, clearer and easier to remember.