If you're writing a client which is sensitive to the resource representation changing over time and you're using something that calls itself an API, hopefully one would skim the provided docs enough to see that you might want to explicitly state the version you want. Or notice that the Content-Type returned for the generic application/json query is a versioned type.

I acknowledge that it's a little more work than just banging out a versioned URI, but it's not that much work and I like the URI/conceptual purity.