(FORTUNE Magazine) – Somewhere in the world, I'm sure there are many thousands of people who invested loads of time and money in getting a master's degree in business administration and who are now glad they did. However, they are not the people who have been writing to me lately. One reader says he used his MBA to line his African gray parrot's birdcage. Others decry the degree as a "joke," the "biggest waste of time and money imaginable," and a "confidence game." In all, the mail is running about 95 to 1 against. Let's suppose you are not in this group. They're just a bunch of soreheads, you say? Think again: Jeffrey Pfeffer is on their side, and he's a professor at the graduate business school at Stanford.

Pfeffer wrote (with Christina Fong, who now teaches at the University of Washington) a scholarly paper called "The End of Business Schools? Less Success Than Meets the Eye" (available at www.aomonline.org). The point is to urge reforms in what graduate business schools teach and how they teach it. But along the way, Pfeffer makes some fascinating points about why getting an MBA is so often just not what it's cracked up to be. "There is little evidence that mastery of the knowledge acquired in business schools enhances people's careers," his study says ominously, "or that attaining the MBA credential itself has much effect on graduates' salaries or career attainment."

Why not? Well, for one thing, there are now so many schools churning out graduates, but demand for MBAs has stayed constant or fallen. Naturally, then, the laws of supply and demand come into play. Even more significant, it seems that most of what people learn in B-schools can be learned just as well, if not better, in other settings. Consider: The supercompetitive job market of the late '90s led top consulting firms like McKinsey and Booz Allen to hire people who lacked MBAs. Those folks (most of whom had only undergraduate liberal arts degrees) got, on average, three weeks of on-the-job training--after which, according to extensive research by the firms themselves, they did their jobs just as well as or even better than their MBA peers.

No one disputes that an MBA from a highly prestigious school such as Harvard, Wharton, Chicago, or Stanford can lead to high pay, partly because of the great contacts students make there. Still, Pfeffer cites study after study strongly suggesting that this is because those schools are so hard to get into (and so costly once you get there), only the best and brightest fast-trackers have a shot. In other words, they are people who most likely would have succeeded whether they went to B-school or not. Overall, the evidence suggests that for most managers a couple of years of extra work experience, not more time spent in classrooms, will pay off better than a graduate business degree.

And what, you may wonder, did Pfeffer's colleagues at Stanford think of his paper? "My dean was not too happy," Pfeffer says, "but he got over it." Ah, the beauty of tenure.