Did you read the article? Blogging is mentioned.

Personally, while I do blog periodically, my journal is entirely different than what I publish online. My journal is for my own consumption. Some things that are written down there (I prefer pen and paper) may end up online or in an essay, but others won't see the light of day (often for good reasons).

For me it's not having things to "hide", it's editing and keeping drafts. And I sometimes like pen and paper to supress premature editing. If I have a keyboard, it's hard to keep my fingers off Alt and Ctrl. (I exclusively use web apps and I haven't found/made a good browser-Vi yet.)

Why is it a bad thing to keep some thoughts to yourself? And I'm not necessarily referring to thoughts about other people.

You are confusing having something to hide and keeping your thoughts to yourself. There is a difference.

No, I'm pretty sure I know the difference. But the older I get, the more sure I am of my opinions, the more educated I am of my thoughts, the better I am of writing, the less I care the more I start to see something to hide and hiding thoughts are too similar.