I agree that internship should be required for licensure, and that it is a critical part of our training. But I don't know if I see why it should be required for the PhD itself. MDs graduate with their doctoral degrees, and then go onto residency. I think a similar model would work for us - if internship is still required for licensure, then you're still getting the same level of training for licensure (e.g., graduate school + internship). But the difference is that the internship is done postdoctorally, and not predoctorally. People who can't secure internships would still be stuck, but at least they would have their degree and could teach, do research, etc. As for the postdoc, well that's an entirely different (albeit related) can of worms.
As someone who is fully licensed after having completed a formal postdoc, I certainly appreciate the benefit of specialized training. At the same time, there is something a bit demoralizing about defending ones dissertation after X years of coursework, clinical work, and research, and not being able to be called "Doctor."