I can only think of a few reasons:
1) Connections. Going to a top school will help you meet and get to know many people who are just as well connected and ambitious.
2) Professors. They can help guide you and give you tips much like a mentor would.
3) Status. It could possibly be easier to get funding and capital if you have a MBA from Stanford on your team.
Is this generally it? To me, it doesn't seem like there's much of a reason to get an MBA, even from a top school, if you're planning on doing entrepreneurship / startups.