I think the author was actually emphasizing the point you made about "why should I care about some hidden feature which I do not use". The 10% of power users he

I think the author was actually emphasizing the point you made about "why should I care about some hidden feature which I do not use". The 10% of power users he was referring to are the ones who use far more than the features you stated. Those features you stated are actually general features which most people use, but these are just a small subset of the vast feature set that is available for users within Word. Most users don't even know these "extra" features exist, let alone what they do and how to use them.