I've got a couple questions on guardian tanking that I'm hoping you all can help me clear up.

1. Relics - it seems like back in the Nov-Dec time frame people agreed that the BiS relics were 1 PvP relic and 1 on-use relic. Now it seems that BiS is 2 PvP relics. I'm assuming this is a result of the EWH gear becoming available, but I wanted to make sure I've got this right. For current gear and content are 2 PvP relics BiS?

1a. My guild is still working on HM TFB, so I'm not really concerned about optimizing for NiM EC Kephess at this point. In fights such as Writhing Horror and Zorn & Toth, am I gimping myself if I equip a 20s on-use relic to deal with the hard-hitting phases? If this is acceptable, is there any reason to favor the Def on-use over the shi/abs on-use, or does this mostly just depend on my stat distribution? So for example if I'm running 30def/50abs I should use a Def; whereas if I'm running 33def/48 abs I should use shi/abs?

2. Def vs Abs - I understand that defense is more important than absorb for guardians. However, I'm wondering if someone can direct me to a post explaining why (or just respond directly, if you feel like explaining it). I'd appreciate any info on this.

On your third point: it's fairly simple.

Vanguard tanks have talents that increase absorption by quite a lot and they can also invest in talents that increases their shield chance by more than what Guardians get. They also have better passive mitigation through armour thanks to talents. Clearly, they're meant to take the hit and mitigate as much of it as possible.

Now look at the Guardian talent tree. Guard Stance and Blade Barricade are both about increasing your defense chance; you have no absorption talent at all so a Guardian's best bet is to AVOID as much damage as possible through defense chance which we can get really high with these talents. Guardians also have the best "oh crap" cooldowns to support this kind of playstyle. Your damage taken is spikier than in case of a Vanguard but when you get lucky you can really give your healer some time to breathe.

Ahh, so because Guardian abilities grant a direct percentage boost to Defense (ignoring DR), that allows us to achieve higher raw percentage numbers than would otherwise be possible. Whereas with absorb, we have no such advantage.

That makes sense, thank you!

Ahh, so because Guardian abilities grant a direct percentage boost to Defense (ignoring DR), that allows us to achieve higher raw percentage numbers than would otherwise be possible.

That's actually the inverse of what you should have gotten out of it. The value of Defense is pretty much static since every increase represents a chance of reducing incoming damage by 100%. The value of Shield and Absorb are directly tied to the percentage values of the other (increasing your Absorb rating by 5% means a 3.25% flat decrease in damage if you've got 65% shield chance, but only a 1% flat decrease in damage if you've got a 20% shield chance). Since Shield and Absorb have such sharp DR curves, unless you've got a lot of native increases to Shield and Absorb (Shadows get 20% Shield and 4% Absorb; VGs get 8% Shield and 12% Absorb; Guardians just get either 0% or 4% Shield, depending on hybrid or full defense), it's simply not worth it to stack a lot of Shield/Absorb.

As such, rather than the massive increases to Guardian Defense chance being the reason that Guardians stack Defense to high hell (Guardians get 12% increase to their baseline defense of 5%; Shadows get 6% to their baseline defense of 10%; the difference is all of 12% better than baseline v. 11% better than baseline; if high natural defense were really the factor, Shadows would stack the living hell out of Defense rather than being really evenly distributed between the 3), it's actually their almost complete lack of increases to Shield and Absorb that render stacking Shield and Absorb mostly redundant. Since Shield and Absorb don't have real appreciable value for a Guardian unless they're largely untouched by the DR curve, the only option left is to stack the holy hell out of Defense.

In short, it's not a case of "lots of defense makes defense good!" (it's true to some extent, but not enough to validate what Guardians do); it's more a case of "negligible Shield and Absorb make Shield and Absorb bad and, by process of elimination, we're left with Defense".

What I meant to imply with my statement [Guardian abilities grant a direct percentage boost to Defense] is that Guardian abilities boost Defense and do not boost Shield/Absorb, which is essentially what you were pointing out in your response [since Shield and Absorb have such sharp DR curves, unless you've got a lot of native increases to Shield and Absorb...it's simply not worth it to stack a lot of Shield/Absorb]. We're both saying that Guardians get little - if any - bonus to Shield/Absorb.

You want to stack the mitigation ability that gives you more mitigation per rating point; and the bonuses granted via abilities and talents directly affect those calculations.

If you have the ability to achieve higher Shield and Absorb without eating into your available mitigation points from gear, that changes the optimal balance point between Defense and Absorb.