There are things you learn while pursuing a MBA that you will not learn anywhere else. Some of those things are highly relevant to being an entrepreneur. Maybe not at the very earliest building stages, but certainly if you make it past that point. Particularly in enterprise focused software companies. That said, there are many paths to achieving your goals - the key is to choose one and stay focused - be it a MBA or YC or any other program. We should all be grateful that any of these options are available to any of us - none of them seem bad to me.