Agree with your exception to the author's statement about usage by people under 20.What I don't get is buying a tablet with an expensive connected keyboard because, if your primary input is keyboarding, why not just buy a laptop with an OS optimized for keyboard and mouse input?
However, a keyboard makes a lot of sense in that it provides flexibility. I don't need it all the time for my iPad but when I want to hammer out an email or several posts on a website I simply attach the keyboard and run with it. When I don't the keyboard isn't there or used as a stand. The problem is that you're equating having a physical keyboard to primary use which isn't the case. Let's flip it around. Why would I want a permanently attached keyboard when I'd use it only 5-10% of the time? Even if I used it more for extensive emailing it still provides great convenience and I don't see an advantage to a laptop in that situation.