Hybrid juggs have the mitigation of a PT but with the Cd of a shadow, but with the least threat of all the other builds. A hybrid has 3 Cd saber ward (50% damage reduction) 3 min , Invisible (40 %) 3 min, Deafening defiance (20%) 45 sec, and 4th if u want to count unstoppable (20%) 15 sec, and a 1 k absorption shield every 9 sec.
Focused Defense (the Guardian Version of Focused Defense) provides only 15% damage reduction, not 20%, consumes Focus, already a high value commodity to the hybrid spec, both at activation and over its duration, and is a threat drop (20% of total threat every second). No intelligent tank is going to use Focused Defense as a tank CD unless absolutely everything else is on CD because of all that: it impedes your ability to generate threat by eating a crapton of Focus and it impedes your ability to maintain aggro by dropping your threat continually over the course of its duration (even if you use taunts, you wouldn't be able to keep aggro the entire time because of the continual threat dropping; you'll either lose threat during the duration or after it's over).
Focused Defense is of such marginal use to most hybrid tanks that I actually tell a vast majority of the tanks I see to simply remove it from their bar so as not to be tempted to use it. Guardians already have threat issues. They don't need those compounded doubly so by a mediocre survivability CD that eats Focus like sweet delicious candy.
This makes hybrid juggs the most sturdy Tank in game.
Hybrid Guards have a survivability advantage over full Defense, but it doesn't really improve them enough to overcome Shadows: getting more Blade Barrier is nice, but it's still nothing compared to Harnessed Shadows (~1000 abs every 9 seconds amounts to ~111 hp/sec; a Shadow should get *at least* 150 hp/sec from their healing mechanisms) and trading out 4% I/E DR and 4% shield for 4% all DR is a great move that makes you take less damage, but not enough to offset the fact that a Shadow still has a well known and well defined advantage in total mitigation (30/65/60 w/ 40% DR and a 5% damage debuff compared to 30/50/50 with 50% DR demonstrates this). The *only* advantage that hybrid Guardians have over Shadows is in the form of a more stable incoming damage profile, of which VGs have an even *more* stable damage profile.
Guardians are not the most sturdy tanks in the game: they are simply the compromise between Shadows (lots of mitigation and active survivability with an unstable incoming damage profile) and VGs (the least mitigation, almost complete reliance on passive survivability, and the most stable incoming damage profile). They are utility tanks, not specialists like the other 2, whether they are hybrids or full Defense.