Seriously, if you really want to go to med school in the US you need to do well on this test. What you are thinking of doing is like taking a cumulative final after having studied only the first two weeks of material of a 10 week course. You will not do well. As older students we often don't have the luxury of taking the test multiple times and applying three or even four cycles. Do it right the first time.
My advice: take all the prereqs, give yourself ample time to study for and do well on the MCAT, and then blow the lid off the test. It is a very doable test, despite what everyone says, i.e. it is not as impossible as people hype it up to be. But you need to be realistic, too, and go into the test prepared. The extra year that you will need to take the rest of the prereqs and study for the test also can be used to beef up your application in other ways, too, so it is not like you are wasting time.
If you just want to take the test for kicks and don't really care how you do (I know people who have done this, and they did not do well <25), then by all means, take the test without the other prereqs.
My two cents.
