Organic Chem definitely makes up a smaller portion of the Biological Sciences section than straight Biology, but with that said, you shouldn't assume that it's going to be tested in X amount. I've heard that it's roughly 75-25 in favor of Bio, but that can be higher or lower depending on which test version you take. I've seen Bio sections with 3 out of 7 Orgo passages which is roughly ~40%. Could you theoretically score well with just prep classes and a favorable test? Sure, but the bottom line is that you will likely be less prepared if you don't complete all your pre-reqs before you take the MCAT.I disagree emphatically with the above posters. I believe one could take the MCAT and score well on the biology section without a day of organic chemistry (it constitutes about 15% of that one section of the test). If you think your biology background is strong, or could be with extensive preparation, then I would go ahead and take it. If you are an otherwise decent test taker, prep courses will give you all the orgo you need to ace it. Will classes make the studying easier? No doubt. Are they necessary? Probably not.
As far as missing pre-reqs at the time of application, I was missing three of my pre-req courses at the time of application and still got many great interview invites. Again, this depends on the overall strength of your application: personal statement, where you took your pre-reqs, in state status, etc.