HBS: Famously, in 2003, Professor Henry Mintzberg (McGill) tracked the performance of the academically top nineteen students who graduated from the Harvard Business School in 1990. Ten of the nineteen were "utter failures," he said. "Another four were very questionable, at best," he added. "So five out of nineteen did well." (2003; NYT: 6.11.06)
N.B. We're talking about a 75% failure rate among the top-ranked students from the world's top-ranked B-School. That really should have triggered all manner of alarms. Unbelievably, it did/has not; it's just business as usual. With tuition, room, board, and (typical) foregone salary, exceeding $250,000 plus 2 years of your career, isn't an MBA a deal?
Worth noting, 2/3 of the NYSE-listed companies are currently run by non-MBA's; clearly, you don't need the diploma. You do need to be able to focus your analytical skills and to communicate in business terms. Also worth noting, Jack Welch holds a PhD in ChemEng but no MBA. It's largely a matter of being able to think and to communicate!
Were all this not pain enough, this from 1992: Take anyones choice for the top ten graduate business schools, eliminate Harvard, sum the headcount of senior executive graduates at "significant" firms of the remaining nine schools. That sum will be less than the combined number of comparable graduates from NYU and Rutgers. Re: ceteris paribus. Obviously, geography plays a large part. Until relatively recently, these two offered the part-time programs of choice for Manhattan's hard-chargers.