I'd only suggest doing this if you're using a responsive multi-column [1] layout. Just because my window happens to be full-screen doesn't mean I want to read lines that are 120 or more characters long.
I actually love HN's design aesthetically, but it just doesn't work in a large browser window (even with significant zoom). Maybe I'll steal some ideas from this book and just make a custom style sheet for it.
Also, one must pay attention to indicate new paragraphs that happen to fall between columns. Without indenting each paragraph, I'm not sure how to make those clear.
I've seen idiotic users use tiny fonts when reading (far smaller that those that are used in books and newspapers when seen from a comparable distance AND with the far worse glyphs that a low dpi screen offers), and others expand their browsers so that they get like 200 characters in each line. They might even "prefer" it and swear by it. It doesn't mean it's better for them or objectively more readable.
One result of this is destroying their reading comprehension, their tolerance for longer texts, and eye fatigue.
Fluid layouts are particularly error prone, looking bad (too narrow or too wide) in all except a few configurations.
Calgo cult individual preferences should take the back seat. With the exception for accessibility for those who need it, designers should target a specific, easy to read font size, line spacing, line width and contrast.