I think EC's are important no matter the applicant, but the magnitude of said EC's depends on your job. If you are a banker, it is probably difficult for you to dedicate a few nights a week to getting heavily involved in some non-profit. However, if you put in 40-50 hours a week, there should be something outside of work that is of strong interest to you and occupies a lot of your free time.
EC's are also important for your essays. When you are describing your experiences, your goals, and your reasons for getting an MBA, it should stem from your EC's as well. Schools want to know that you have a life outside of work (as much as possible) because they want someone who will contribute in other capacities besides academics.
EC's differentiate applicants. Since you started college, most b-school applicants strive for a good GPA, a good job, and now a good GMAT score. As difficult as this is to achieve, there are scores of people applying to top schools. So the top schools need to look at something else that separetes those who go for just the trifecata and those who strive for a little bit more--despite how busy they are with the other three elements.
Whoever said it is not too late to get started with EC's is absolutely right. The most important advice on getting invovled in an EC now is make sure it is something you believe in. Of course, it is easy to write about something that you have been doing for awhile, but if you are not enthusiastic about it that will come through in your essays. So, if you are applying now without any EC's, it is time to get involved. Inventory your interests, gauge your time commitment, and choose accordingly. It will make a difference in your app.