Most students approach their undergraduate coursework with a test-of-the-week mentality, so when the time comes for the MCAT there isn't good structure to the knowledge-base. Folks go through physics one chapter at a time but never step back and actually read the table of contents of their textbook and say, 'okay, physics is mechanics, waves, thermodynamics, electricity and magnetism, light and optics, modern physics etc.
So I think it's really very good that you are thinking about your MCAT at this stage before taking your undergraduate courses. I know many students who would have benefited to have taken a bit of time to familiarize themselves with the exam before the prereqs, at least to have some kind of consciousness that they were building a knowledge-base geared to performance on a comprehensive, conceptually oriented exam in the future. As other commenters have noted, it's probably not very useful to spend a great deal of time on explicitly MCAT related tasks, it causes your coursework to suffer, but researching the MCAT and addressing MCAT related learning goals, especially as relates to overall perspectives, could definitely benefit you.
