Right.
First off, pixel fonts were the "hype" in UIs for quite some time. Everyone needed them. Then there was a period of the opposite; anyone who had pixel fonts in their UI was obviously just following the hype.
Nowadays it seems a bit less one-sided.
I've used pixel fonts in my UI ever since I made it (which is a year and a few months ago, and it was rather awful at the start) and I've never switched based on how popular they were. Though, the idea to use them in the first place probably came from its popularity.
I just think pixel fonts, together with 1-pixel outlines, add a "crisp and clean" feel to a UI that is very hard to replicate with regular fonts. For names and numbers, e.g. on unitframes, it's perfectly fine, but in the chat frame I prefer a normal font simply because it's easier on the eyes.
Tl;dr: author's choice.