This increases the value of the certificate as proof of knowledge and competence. Let's imagine that MIT offered 20 engineering courses in this format. You go through the effort of honestly taking these courses and add them to your resume. A potential employer says:
"Well, these courses are really rigorous -- much more so than those for-profit online 'universities'. While MIT will allow me to verify that a single online account holder claiming to be this person completed these courses, there's no proof that you didn't cheat or pay people to take the tests for you. Just look at how many CS homework projects are listed on Rent-a-Coder! I have to bring this person into my office and check them out myself to be sure."
I have argued previously that this "checking out" process isn't so expensive that an impressive resume would be passed over, and I still think this. However, what if a student had the option of paying a fee to take a test at an approved testing center which would check identification, take a photo of the student (available on MIT's site along with the course certificates), and ensure that this person took the test himself? Those taking actuarial exams and such already take tests at such locations. For those who didn't want to pay, lived somewhere without such centers, etc., certificates could be given which would have a lower level of authentication. If you are taking the time to complete a course and want to prove to others that you did not cheat, wouldn't it be worth paying an extra $100?