The answer to this question is actually super obvious, though:
The Annulet is good for specs that have low base damage and/or that only really scale with Crit/Versatility and avoid Haste/Mastery like the plague. Annulet is just a damage proc that does base damage scaling off Crit/Versatility, as is the case with the Eranog ring, Bomb Dispenser, Beacon to the Beyond, etc.
Note how the specs that like secondaries, and that consequently scale aggressively as their gear gets better and they get more Haste, (Unholy, Devastation, Shadow, Warlocks, etc.) usually replaced the Annulet reasonably quickly. Now, Warlocks still admittedly use their Eranog ring, but they can also consistently proc it which gives it a ton of value. Now, note how the specs that dislike secondaries (Ret, Sub, MM to an extent) are still using it in many cases and how they’re falling behind quite hard compared to those aforementioned well-scaling specs.
Blizzard knows this is an issue, and most of us know it’s an issue too, but they can’t really address this issue without soft or even hard reworks for these classes. Ironically, the Ret rework made them not want their original two best secondaries anymore, which is the one failing of this spec in my opinion.