I doubt the skill of test taking is unrelated to aptitude in those fields, or any other field in which it's important to learn a great deal of information from books and to keep that information in mind for rapid recall during high pressure situations 'in the field.' Law or medical school is certainly harder for halfwits to get through, but that doesn't mean there's anything wrong (in that respect) with that schooling. Rather, it reflects the complexity and importance of those fields.

Edit for response:

I don't know anything about the MCAT in particular. However I'm confident in saying that the class of people unable to perform well in any sort of test that requires reading and recalling large amounts of information (as learning medicine surely requires, no matter how you structure the education system for it) should find a career outside medicine.