Business school is not “all about contacts”. All your school contacts will graduate the same time you do into jobs at similar levels. Ten to twenty years from graduation when you’re actually in a position to help each other, how many classmates do you think you’ll still be in contact with. How many of your classmates are you in touch with now?
Second, you do actually learn real stuff in business school. Do you already know accounting, finance, economics, business law, marketing, management, and business strategy? If so, you probably don’t need an MBA. But I never had those courses in undergrad, so for me an MBA wasn’t a waste of time. If you are going into business, you should learn how business works.
People who go to top tier business schools like Wharton or Harvard tend to have a bit higher aspirations than “tidy six figure income as Director of Sourcing for the equivalent of a Fortune 500 company.” Many aspire to C-level positions, top tier management consultancies, investment banks, hedge funds and private equity firms. Jobs that can pay seven or eight figures or more.
That said, most MBA grads will probably go on to jobs in the $150,000 to $200,000 range 5-10 years out of school. If you can figure out a way to earn that kind of money without an MBA, then by all means, have at it.
Business school is similar to law school in that it’s not just about getting a professional credential. It’s about fundamentally changing the way you think about your career. In particular, it’s to train you to be a business leader.
Like anything else, there is a hierarchy and a bit of elitism. In addition to the actual rankings, full time programs are generally considered more prestigious than part time. Like the consulting firms and investment banks that hire out of Wharton or HBS want the sort of kids who can afford to take 2 years off from working as analyst at whatever investment bank or consulting firm they worked at out of undergrad.