I do it for sizes less than 10KB (completely arbitrary) for most websites because the fact of the matter is that most websites don't get repeat visits often, and visitors rarely visit more than the page/article they arrived at.
There's no point making all visitors cache a CSS file when only a tiny minority will need it.
Note that I live in the middle of the Pacific Ocean, so HTTP round-trip times are terrible for most users. If you're on a first-world connection, it shouldn't matter as much.
It doesn't necessarily have to be about size. The goal is to let the browser paint the above-the-fold content without any external round-trips. Using the least possible seams sensible, especially if you do it on every page. This lets caching work as intended for the majority of the CSS.
