"Good enough" is not a term that applies to MBA admissions. Nobody knows whether a 700 or 740 or 780 will be good enough. The applications process is not a yes/no question. For the most competitive schools, you need to maximize each part of your application to have a chance. Even if your score is above the average for a school, there's no guarantee that you will be admitted. A higher GMAT score won't necessarily get you admitted, but it could help.
There are other reasons to get as high a score as possible. The first factor is money. For most schools, GMAT is a key factor when awarding scholarships. I'm a perfect example of this. Also, if you're planning to look for a job in investment banking or consulting, the top employers will all ask for your GMAT score. The prevailing thought is that if you are below 700, you will have some explaining to do when interviewing at one of these companies.