You meet someone. They're kind of cute. Maybe they were sitting next to you in lecture, maybe it was someone at the gym. You get their phone number and later decide to send a text. You spend some time on the message: not too long, not too short, reminding them of the conversation you had and ending by asking if they would want to grab dinner some time. After editing it six times, you take a deep breath and hit "send." You watch your phone, but nothing comes that day. Or the day after that. Or the week after that. You finally get a response four weeks later. "Hey, I remember you. I would be happy to meet."
How interested is this person in you? Maybe they have a good excuse, but it's pretty clear from their actions that they don't care that much about going out with you.
This is how med schools feel when you turn their secondaries in late. They really want to like you, but it's hard if you don't show them some love back. Med schools will only interview students they think would choose them. Just like dating, you should treat each of your prospective med schools with respect.
Taking our metaphor a step further, your secondary should be a love letter to the medical school. It should be colorful, unique, and timely. If you send your secondary back four weeks later, the med school will know that there are other schools you'd prefer to go to, and it will usually reject you.
Why does this happen to so many pre-meds? You get overwhelmed. If you apply to 30 schools, you will receive approximately 10 secondaries (with roughly 3 essays each) in the first week of July. The next week, you get another 7 secondaries, and more in the weeks that follow. And when you suddenly have that many essays to write, you get stressed out, and you have to prioritize which schools to respond to. This means you will respond late to many schools and, even worse, you will not respond at all to several others. This single mistake could be responsible for you eliminating your chances at half of your schools.
How do you avoid disappointing half of your med schools? Pre-write your secondaries before you receive them. The prompts don't change that much from year to year. You can find them online. In fact, Passport Admissions has compiled every secondary essay from every US medical school we could find for the 2013-14 cycle.
By pre-writing the essays before you receive them, you help make sure that your respond in 4 days instead of 4 weeks. And then maybe you will get to go on that date, and if it works out, you will get married to the med school (for four years, at least)!