The autoblocker really isn't for paladins. It's a warrior threat trinket. Save your badges and try to get your hands on the Commendation of Kael'thas and Moroes pocketwatch.
It's extremely nice for any kind of AoE tanking, especially at the T4-T5 levels. If you're right at the progression gear level for tanking murlocs on Morogrim, then the equip effect can easily amount to a 10% net damage reduction (assuming you have Redoubt, naturally). Alternating the use effect with Figurine of the Colossus on consecutive waves can make your healer's life a lot easier.
I don't recommend it as one of your first badge purchases, but it's definitely handy to have.
My comrades are my weapons, and I am their shield.
I have to take the opposite stance here, the Autoblocker was one of my first badge buys at 70 and I absolutely recommend that everyone pick it up. If the BV is 10% damage off on Morogrim it's far more in Heroics and Karazhan. The exact numbers depend on a lot of factors, but if you're [near] uncrushable, anything that hits under ~2k damage probably gets more effective health from the Autoblocker than a stam trinket.
gives you the effective health saved, which you can then use for comparison purposes (obviously take out coefficients as necessary if don't have the meta/shield spec).
So for example, if you have 18k raid buffed and the average incoming hit is 2k, the [Item not found!] will give you 9 x 1.43 x 59 = 759 health (discounting the use effect) or 9 x 1.43 x 92 = 1184 health (averaging out the additional block value from the use effect). The averaged out figure is a bit deceptive (given that the mitigation it provides is only present for 20 / 120s) so I'd stick with the discounted value myself.
In comparison the [Commendation of Kael'thas] gives 1.06 (Sacred Duty) * 1.1 (Combat Expertise) * 1.1 (Kings) * 57 * 10 = 731 health (and a passive effect which is not accounted for here, but which obviously makes the Commendation a better choice overall).
Note additionally that BV is less valuable if HS drops off (crushings) or you are taking magic damage, whereas stamina is always effective.
For most boss fights you will of course get hit for a lot more than 2k on average, so stamina trinkets will generally be the better choice.
After talking with my guilds paladin tank about trash equip, we figure it makes sense to set up the trash equip one uses to have a total avoid of 101.6% (Mobs gain 0.2% anti-dodge/block/parry/miss per level above the player level), since trash mobs have a max. level of 72 you don't need the full 102.4% and can invest those stat points into more stamina or spell damage, while still preventing normal white hits(i.e. only having blocks). Since I saw no one else post about this, I figured I'll contribute it.
By the nature of trash tanking though, you might very well take more hits than holy shield charges.
And even if you're not, the benefits to gearing for blocks vs hits is a dubious one at best. Even in a high-block set, you're only going to be saving ~700 block per hit, and that's generally at the expense of stamina and real avoidance.
Block only really shines in multi-mob tanking environments, and like Vorda said, gearing to not be hit there is sorta useless as you'll likely not have holy shield/redoubt up all the time.
I wouldn't even consciously gear for unhittable on bosses if they didn't crush. It's just that the randomness factor of really spiky damage makes it necessary.
While on the subject of the auto-blocker, it's got me looking at my own itemisation set-up for Felmyst. As covered a page or two back, the incoming damage is pretty trivial of course, but I don't mind chugging more mana pots if it means our healers save some more mana.
I, like I would imagine most would, use Figurine of The Collosus for the use and the extra blocks in general. Previously I've used Dabiri's aswell.
In the general case for predictable low dmg from multiple sources I guess there must be some kind of relationship between frequency of incoming hits, size of the hits and the amount of block value/% you already have that can be derived to compare the Auto-Blocker and Dabiri's.
I'm not fantastic at theorycraft, but I'll give it a go and you lot can correct my shocking logic/maths.
You could use the frequency of incoming hits to derive Redoubt/Holy Shield up-time to work out the number of incoming hits a second required to ensure that the full use of Dabiri's is well, used (15.85%), or how much of it would be used (this is assuming that without redoubt/holy shield you have more than 17.88% [15.85 + 2.03 passive] "to be hit" on your combat table, I fair assumption I think).
Once this is derived you could work out how many hits per time (the time being arbitrary as both trinkets have the same up-time a minute of 10seconds) blocked due to the additional block rating and add to this the 2.03 of total miss/parry/dodge/block gained from the passive defense on the trinket.
This can then be multiplied by your current block value to find how much damage per time is reduced from the trinket.
As for the Auto-Blocker, we're looking to find out how many times you currently block per time, and how much of the passive/use block value is.. used.
To work out number of blocks you would add the passive block from gear (minus level difference of mobs 0.2% per level above) to the Redoubt/Holy Shield up-time worked out previously.
Then you would need to minus the current block value from the size of the incoming hits (after armour etc etc etc is applied) to see how much damage is available to be blocked. If this is more than 370 (259 x 1.1 Metagem x 1.3 Shield Specialisation), then the full usage is available with the use and you get an average additional 131 dmg per block mitigated. If less than 370, the amount less minused from the on use amount of block value multiplied by 20/120 will bring about the average additional dmg per block mitigated (up to the point of the on use effect is actually not being used, I don't think that 59 block value could ever be better than 2% avoidance in an aoe circumstance for pure total damage reduction.)
Therefore multiply the number of blocks found previously to this additional damage reduction per block to find the damage per time reduced from the trinket.
I can do the maths on my own gear in my own time, firstly:
1. Correct my maths/logic.
2. How do you work out Redoubt/Holy Shield uptime from frequency of incoming hits?
3. How do you work out Block Value given Tome of The Lightbringer?
All depends a bit on what fight you're on.
Some fights you need more armor/stamina to simply give you a bigger pool of health to absorb damage in.
While on other (spiky) fights you prefer to have more general avoidance to create potentially longer gabs between damage spikes coming in.
Why don't you keep the 2 trinkets and the 2 boots in your bags and switch according to the fight?
I know I have some extra survival gear and some extra spell damage gear in my bags to alter for groups with weaker healers with average DPS or strong healers with strong DPS.
On my hunter I go even further by bringing gear for the case we have a moonkin (3% hit), in case we don't, in case we need lots of sta or lots of int, and then there is the SR gear for mother.
Bag management and maybe an extra bagslot in your bank is all it takes.