Say what you will, the world looks at you differently as an MBA. CEOs and senior management at global multinationals tend to be MBAs and tend to hire and promote other MBAs. What is it about this degree? The academics are pretty basic, undergrad stuff. Doing case writeups probably has some value in terms of writing, time management and presentation skills- but it is far from clear that doing case studies makes you a great manager- especially when technical skills (accounting, finance, computer programming) are not really taught or basically glossed over.
The only thing I can think of is signalling and supply/demand. There are many people who want to work as brand managers at P&G or do banking at JP Morgan. The MBA is a filtering mechnanism to lower the supply of potential candidates for coveted corporate training programs. Even within the MBA world, there is a lot of rankings hierarchy- H/S/W, M7, top 16, etc. Even top MBA students tend to be surprisingly immature, intellectually shallow and harbor a weird superiority complex.