I'm a developer at a financial services company. I was content to be a developer and let the "business" folks define our stories and I'd just code them. I never understood the application I was building on a deep level--it was always superficial. A friend convinced me to take a few MBA classes at a local university. The first class was accounting. It opened my eyes to understanding what our product and business owner…
Most entrepreneurs need to spend a significant amount of time thinking about non-tech related objectives (e.g. sales, pricing, marketing, etc). Sure -- there are tons of publications out there teaching a technical entrepreneur the fundamentals of marketing and sales, but it is always specific to the author's/interviewee's own experience which happens to be unlike all others. Or, and perhaps more commonly, these publications tend to gloss over a really important fact (or facts): the author/interviewee had insane connections or a very rich uncle that allowed her to achieve success as an entrepreneur/businessperson without an MBA.
For those would-be entrepreneurs without insane connections, deep pockets, or a future unicorn in the making, the MBA provides an opportunity to build skills and knowledge important to getting a business off the ground and, more generally, teaches one how to operate in a world that revolves around money, not tech. Personally, my MBA gave way to a great new career path, higher salary, great connections, and -- most importantly -- a much more holistic view of the world.
In the end, I do believe that an MBA is not required for success. However, I disagree with Thiel and Sandberg drawing such a hard line in the sand. People looking to learn is never a useless act.
For context -- I picked up an EE degree in undergrad and eventually went on to grab an MBA.