To piggyback on what some others have said, I don't think having a sub-700 GMAT necessarily precludes you from getting into a top school. That being said, if it is sub-700, then there will need to be something elsewhere to make up for not having the requisite GMAT score, whether it's an extremely high GPA from a challenging school, a very impressive work experience/LOR, etc.
the GMAT is the one element that all applicants have in common and the only element that can't be easily faked. You can BS your resume, you can write your own recommendations. You can hire someone to write impeccable essays that have almost nothing to do with you. You can use connections to get a job at a good company. Even GPA varies widely from school to school, where some schools average a 3.4 GPA and others average well below a 3.0. The GMAT is the only element of the application that can't be faked.
Is it the best indicator of how successful you will be? Of course not, but it does test your ability to learn a relatively finite amount of content and be able to process that content effectively in a stressful, time-sensitive environment, which is not unlike what happens in the business world.
The GMAT sucks. That much is certain. At the end of the day, however, is it a necessary evil.