Hey, I addressed this point in the blog post. I disagreed with that because a) its not self-documenting. b) On v2 the exact same URLs will have completely different data and probably break every API consumer.
It's like this:
--> GET /api/ Accept: application/json
<-- Content-type: application/vnd.api.v1+json <stuff>
Then your consumers can know what version of the API they're interacting with and act accordingly.
Including the version number in the URL gets around this and even allows you to test API responses in mobile browsers if needed.
Although I tend to agree that a header-based approach is cleaner and more in the spirit of REST and even HTTP.