1. There are always outliers - those who take the MCAT as late as September and still get a seat the following July / August... but they are outliers
2. Taking your exam later than the rest of the people does a few things:
a. as interviews are sent out on a rolling basis for many schools, many of those invites will have been sent and possible acceptances sent before you even are considered complete
remember, primary has to get validated by AAMC first before being sent to schools; July and August are considered the heaviest months for validation; no school even knows you exist until that piece is done by the AAMC which can take up to 4 weeks to complete; now you are into August before getting into the pile for secondaries which means possibly September for full review by adcom at schools
b. puts you into the common bucket rather than the "was ready to go at start" bucket... which is easier and more likely to be reviewed; not all applications are reviewed (I've had that confirmed by adcoms at schools that get inundated by apps... )
All that said, if you are NOT ready to take it, do not take it. Do not take this exam until you are confident enough in yourself to sit for it. That means you've completed content review, done enough passages to get a feel for the exam, done enough full length exams under timed conditions to feel ready (no one ever, or very rarely ever, says, "Got this - gonna bang out this MCAT thing." most do not feel like it before or during and certainly not after until they get their scores)