A lot of folks spec just high enough into the tank tree to get the majority of the defensive abilities (basically up to sonic barrier) and then spec vengeance up to impale. They do this because the top talent in immortal, crushing blow, was somewhat mangled in the last major jugg/guardian update - the overall damage was reduced and it was turned into a weird cone AoE. Theoretically this was to help with AoE threat, but the cone is narrow enough that I find getting more than a couple targets in it is pure luck (speaking as a jedi guardian here - my jugg is still only L33).
The mixed spec has the advantage of a hard hitting single target attack (impale) on a 9s recharge instead of the 15s for crushing blow, unstoppable, which gives you 20% extra damage restance and immunity to CC for 4 seconds after a force leap, free smash and 4% extra damage resistance from deafening defense (up to 15% extra if you can fit in enraged defense without shedding to much aggro). You give up 4% shield chance, which is not a big deal on a jugg since they rely more on defense and damage reduction, 4% internal and elemental resistances (which deafening defense more than covers) and your rage generation/usage is not quite as good. According to those who play this spec your aggro generation is about equal to the full defense spec as impale may not have increased threat gen but it does more damage than crushing blow and is on a shorter recharge, so the net aggro generation is roughly equal.
Speaking personally, I tried the spec on my jedi guardian and didn't really like it as I had to much trouble with focus use/generation and was always running out. Now, I only tested the build in solo play on ilum, so it might work better tanking in a flashpoint or operation, but I switched back to the full defense spec and enjoy that a lot more.
Erichough is more or less correct. I leveled as a full Immortal juggernaut and played that way until 1.4 came out when I switched to hybrid and have not looked back. One important thing he missed is that hybrid spec reduces the cooldown of Force Scream by 25% allowing you to benefit from Sonic Barrier more often.
So the hybrid has better survivability and single target damage, essentially the same single target threat at the cost of worse rage generation, AoE damage, and AoE threat.
I don't like the full Immortal build because the top half of the tree exists primarily to offset the uniquely
high resource penalty of our tanking stance while not offering much in the way of survivability or treat/damage gain.
Also I find the current implementation of Crushing Blow horrible. It barely does more damage then our default rage dump at a higher cost with a cumbersome AoE tacked on. To proc the AoE you need 5 stacks of the armor debuff and the only efficient way to place those is to use Smash followed by Sundering Assault making it an AoE that I can only use quickly on targets I have already hit with my main AoE.
This causes two problems. It makes a joke of Crushing Blow not breaking CC because Smash will. Also in most of the fights where I'd want another AoE it is because the enemies are spread out and Smash won't get them all or they come as adds part way through the fight and I rarely found Crushing Blow useful in those situations.