This myth is so widely repeated I have to call it out. The people you meet doing an MBA either a) leave and go into corporate middle management or b) go start their own businesses. The chance that the middle managers end up in a position where they can directly or indirectly help your startup is extremely low; the idea that you could be fully occupied as an entrepreneur just exploiting those contacts for 12-24 months is laughably naive. As for your MBAers who become fellow entrepeneurs - well they have their heads down working on a startup which is probably in a totally different sector to you - they're incredibly busy building their own businesses and contacts in a totally different sector, and even less able to help. Overall, best case scenario is that an entrepreneur starts a B2B business and a handful of his classmates end up as potential clients/introducers to potential clients - but even with five of these contacts, at $100k for an MBA that works out at $20k per lead, hardly the best return on investment. There are plenty of reasons to do an MBA, but networking to establish a startup isn't one.
It depends on the school you attend. The networking effect is bigger than just the people you attend class with, although those connections are often very valuable. Most MBA students aren't studying for entrepreneurship. They're someone who has already had success in the business world and are going to advance their career. Having a network of connections across business domains can be very valuable.
Obviously it depends on the startup. B2B businesses would get more value out of those connections. I stated that it's likely the biggest benefit of business school, not the only benefit. A true cost-benefit analysis would look also at the knowledge acquired as well as the strength of the alumni network.
