I also run the full immortal spec juggernaut, the survivability is minimally worse but the threat generation is so much better with my rotation that I find its better for the group.
I highly doubt the threat generation is much better. I'd like to know what your opener is like, I could easily see an opener from the Hybrid spec doing more damage and threat.
In the first 15 seconds of a fight, a full Immortal will get off 1 Crushing Blow, a hybrid will get off 2 Impales, and Impale actually does the same amount of threat as Crushing Blow, if not a bit more, because of how much more damage it does. You also get more Screams in (your third best threat move as a Jugg because of the damage it does), and the damage from Backhand/Hilt Strike is pathetic enough that its high threat modifier doesn't even really matter, only makes it about on par with a Scream or Smash.
The only real advantage for an Immortal Jugg for snap threat early on in a fight is 20% more damage on Smash. But being able to use 2 Impales before an Immortal can use 2 Crushing Blow's, or 3 Impales in the time Immortal gets 2 CB's, is a rather big advantage for the Hybrid spec for snap threat. Not to mention Hybrid Jugg gets 3% accuracy from the Vengeance tree which helps to make sure Impale hits (no help on Crushing Blow for Immortal), and it also gets a bleed that does about 20% of the damage Scream does tacked onto it.
The only area that I do think Immortal can shine is in long term sustained damage/threat, because of how much better their Focus/Rage management will be over the course of a long fight. But sustained threat is never an issue in TOR because of how taunts work.