When you're founding a startup, your team needs to have certain skills spread between the members. One is sufficient technical savy to put together a product (engineering, graphic design, product design, programming, etc.), and the other is sufficient business acumen to make people buy that product (market analysis, marketing, accounting, salesmanship, negotiation, planning).
There is this belief often held with engineering heavy crowds which is "Build a better mousetrap, and they will come". They won't. Your potential customers won't suddenly feel a tingle in their stomach leading them to the store in order to buy your mousetrap. That tingle needs careful engineering as well, as well as getting the mousetrap into the stores, handling the payment processing, paying the manufacturers, you get the picture.
Similarily, there's the belief in the marketing crowd that "I can sell you anything, I just need a product". It's equally flawed, out of obvious reasons.
So obviously engineers want nothing to do with slimy business people, and businesspeople want to have at least 3 layers of organization between them and the filthy engineers. So if you can step into both pairs of shoes, you can bridge that gap and you're at a distinct advantage.
There are however severak courses that I'd give a shot out of the list Sloan makes public. Sytem Optimization Network Optimization Early Stage Capital Finance I&II System Dynamics A whole slew of Accountig & Law Courses
As you can notice, I left out anything marketing related, current affairs related anything industry specific and anything ethics related.
It's not that I think those are useless, because I agree with you. Business school marketing will teach how to talk about marketing, but not how to do marketing. Current trends are better analyzed by yourself, and industry specific knowledge should be gained directly from the source. Go talk to some people who are in that industry. And if you don't have personal ethics, business ethics isn't going to help you, and if you do have them, business ethics is not going to teach you anything beyond what you have.
Business school covers a fraction of doing business, so you have to supplement it with other sources of knowledge (Marketing? Get your fingers dirty with PPC. Salesmanship? Sell vacuums over your summer break). But if you have most of your trouble with that fraction, get help with it even if it's just formally learning the stuff.
So business school has limited utility, which should not be simply discounted.
And yes, you can study everything on your own. But then why go to college at all?
Don't get me wrong--I think those core accounting and finance classes are very important. I personally wish I had taken some more of them while I was in law school (we had them cross-registered with Kellogg), because it's relly valuable, broadly applicable stuff for anyone running a business or dealing with business clients. But a) you can self-teach that stuff as easily as you can self-teach engineering; and b) that stuff is a necessary but not sufficient subset of skills you need to be a great manager and leader.
That's my biggest beef with business school, especially at the undergraduate level. I think a good engineering school serves as a reliable indicator that someone can do the work. But with an MBA, you need to do more to see if the person has more than the corporate finance and accounting skills they learned in school.
* marketing
* accounting
are all required for a successful and effective business. Of course its possible to learn these on the fly, but why not sponge up some knowledge without taking the high levels of risk involved in starting your own business.