There is nothing wrong with deferring. It depends on a lot of factors. If you're older, married, children, why wait? But if you are younger, there are many good reasons to wait. Maybe you're not fully comfortable with your motivation in medicine, want to take a masters thesis in public health or hospital admin, or become an EMT or volunteer abroad and develop your altruistic side of being a doctor. Maybe you're out of shape and always wanted to train for a triathalon or like to travel and camp and know you won't have the opportunity to do it for a long period of time within the next 8 years. Or you might want to go to queens to find a bride. I wouldn't just wait a year without a plan or good reason, but there are plenty of good reasons.
Opportunity costs work in both directions, especially if after the long journey of medical school, you are unmotivated and feel you missed a good opportunity first.