I think it depends on the person and the school. You can teach yourself anything if you really want to, even surgery, but I wouldn't advise trying to practice it without a license. And going to a school doesn't change the fact that you need to be dedicated to your goal in order to achieve it. If you go to a school, you will work just as hard, or at least you should. It's possible I suppose to get through school without working as hard, but your skills will suffer for it.
I totally believe that you don't need school to learn anything, just like Marko, but I also think that a good university can provide some things for you that might be harder to come by on your own.
1. Feedback and motivation -- Not just from your teachers but from fellow students. And you're not just getting feedback about the end results. You get to share thoughts on techniques and ideas in the early development stages in many classes. You get feedback on the process as much as the final result and the process is just as important to the professional designer as the finished product is. Good teachers also push you harder than you would push yourself. Scott Robertson is renowned for pushing his students to the brink of exhaustion in his drawing classes at ACCD. My drawing teacher basically demanded huge volumes of work to succeed in his class, and constant improvement had to be demonstrated every week. I must've spent twnety hours a week just on that one class. I'm pretty self-motivated. I've taught myself over a dozen programming languages, business management, Japanese, philosophy, theology and god knows what else. But even I wouldn't have pushed myself to draw that much in that time period were it not for my teacher's cruel demands.
Maybe you're one of the self-motivated learners who doesn't need a cattle prod to stay focused on work. Some people do need a prod once in a while though. Be honest with yourself which kind of person you are if you really want success. Don't try to do it on your own if you don't have the temperment and discipline to pull it off. Be honest with yourself if you're serious.
2. Competition -- It's easier to motivate yourself to do your best when you're surrounded by other students who are doing the same. This, of course, implies that your fellow students are strongly motivated, which depends on a lot of factors, but at a good school, it's usually the case.
3. Resources -- You're thinking about industrial design and not just concept design so pay attention to this one. What does a university have that the average design student doesn't? Machine shops equipped with mills, lathes, rapid prototyping machines and maybe even plasma cutters (*lust*) among other things. When studying ID, you need to learn about materials, and the best way to learn is to build something once in a while. If you have your own shop (like I did because my dad is a machinist), then you don't need a school for this stuff. Otherwise, consider it. Also, think about the computer labs filled with high-end graphics workstations loaded up with the latest 3D software. It's nice not to have to pay for this stuff when you're still just learning. Of course, the quality and availability of these resources again depends on the quality of the school you go to.
4. Faculty -- Classes are one thing, but it's also nice to have a professor in the metal shop who can give you instant feedback on one of your designs. You can ask them questions like, "How would I go about building this?" or "Can I build this?" Most ID schools give their students pretty good access to their shop facilities for school and personal projects. Having those shops staffed with people who are experienced and knowledgable in their use only adds more value to them.
5. Networking -- It's been said before, but getting a good job in the design industry is about who you know as much as it is about how good you are. One good way to meet people is in school. And not just students. Professors have ties to the industry usually and can help you out if you distinguish yourself in their classes. Schools attract guest speakers and lecturers, most of whom are very approachable and can be downright friendly when offered a free beer. Student chapters of IDSA are incredibly valuable because they offer all of the benefits of a professional chapter at a fraction of the membership cost.
6. Your portfolio is your resume. A good portfolio can get you in most doors. But product design is about more than good pictures, there is a function implied in every designed product. And they have to be manufactured. Industrial design is more than just drawing basically. A good ID program includes drawing/art, practical knowledge like materials courses, some science and engineering (though most go pretty light on this stuff, two or three classes tops), some psychology (human factors mostly) and maybe even some marketing. Good industrial designers have a lot of varied skills and employers want designers with as many skillsets as possible these days. The rennaissance man is back in fashion in industrial design. Your portfolio can tell an employer how well you draw or what software you know how to use for 3D renderings, but without that four-year degree under your belt they have no way of knowing what you know about manufacturing processes, materials, human factors or product marketability.
Again, this all depends on the school. Go to a good one (if you go to one). Find an accredited one if you can (ID is one of the few academic areas where accreditation actually means something). They don't have to be expensive. (ever thought about Metro State in Denver? It's a great program at a low price)
So yes, you can learn everything you need without ever setting foot in a classroom. And yes, you can also learn everything you need by going to a good school. Neither approach is necessarily better than the other, but one might be better for you. In my experience, most people benefit from school, but I've met self-taught professionals who didn't need school at all -- they really are the exception more than the rule though.
Personally, I'm glad I chose school. I think it accelerated my learning process, but I put a lot of effort into making it worthwhile. I also have a stack of training DVDs and books. They're all good resources, you'd be a fool not to take advantage of every resource you have access to. Use them all. Take advantage of every opportunity. Take any step you can.
The most common person goes to school, doesn't dedicate themselves to their studies and ends up dropping out or just sucking at their chosen profession. No one should think school is a shortcut to avoid hard work and dedication, but far too many people seem to think so anyway. But don't let that be an indictment of school in general. It can be very valuable for the people are willing to put their all into it.
In the immortal words of Socrates, who said, "I drank what?"