An MBA is good for two things:
a) it's is a great way of formally recognising the skills you might already have
b) it FORMALISES your method of work.
So (a) If you are starting out on your own, you will work it out through your own mistakes, however if you need to seek funding, it gives you credibility in the eyes of others.
In the same way; if you're looking to bring someone on board with you, and you both have MBAs, you know that you will both be "on the same page".
Plenty of successful entrepreneurs will not have MBAs - or even any business qualifications at all!