My guess is that Christians rarely refer to the watchers because the term is not used in the New Testament. I just looked at the Septuagint, the Greek versions of the Bible that the early Christians used, and the term "watcher" isn't used. It appears that the Aramaic word "Ir", which means "watcher," was simply transliterated or even translated as "angel."
We probably see a lot of the watchers in the Book of Revelation, but the term isn't used and instead they are called angels.
I looked up this **Apocalypse of Yajnavalkaya** and it looks intriguing (except for that "Quaternity" bit; what is that, a substitute for the Trinity? There aren't any openings in the Trinity, as the old joke goes).