The traditional thought is that career switchers need a 2 year program so they can explore new jobs during the summer internship. This is especially true for IB jobs because they traditionally hire most of their full-time people from their summer programs. They might "fill out" their classes with a few additional people each year, but the bulk of the hiring is done with 1st years that go through the summer program.
Jobs in other industries might not be so tied into the summer program. Just remember, if you are participating in a 1-year program, your pre-MBA experience will be a key element in your job search. This is not a problem if you are staying with the same company or in the same industry, or if you are starting your own business. But if you are planning a career change, it will be much tougher.