There are plenty of things a Chromebook CAN'T do, that's true.
But, nearly everything the average non-techy person does with a computer can be done with a Chromebook. And there are a surprising amount of very useful apps, all of which you can try out beforehand by installing the Chrome browser on your PC and checking out the Chrome store.
The advantages of a Chromebook are:
Lighter weight
Often better battery life
Boots in just a few seconds, literally.
OS upgrades happen automatically, in the background. Most people never realize it happened.
Essentially free of malware.
My niece has one that she uses for school (high school). Most of her homework is submitted via Docs/Gmail, some schoolbooks are ebooks, group projects are done via Docs (including Presentations), and of course, web browsing. She has a Windows laptop that she probably used twice this year for things she didn't know how to do on the Chromebook, and she hated it, because "it takes forever."
Clearly, a Chromebook isn't the right computer for everyone. Myself, I'm a heavy computer user, and I do a bunch of things the average person never does, but even then, I can run my business from a Chromebook with ease, and they are small and light and easy to take with you, and quick to bring up things in if you need to show a client something. I suspect most people buy them as secondary computers at first, but they probably find themselves using the Chromebook 75+% of the time, just because it's so easy and convenient. If you try one, you might easily be converted...