Both cases you've mentioned are so marginal that I'd rather pop the KJC active especially on Stone Guard. 6s of MG (worth >200k for MG alone) while moving and running tiles on is worth far more than AV during Chains.
I've tried both using AV on high damage sections of a fight and keeping it on passive the entire fight. It does nearly identical damage either way, so there's really no point in risking death for what literally equates to a 200 dps increase.
List ways you use AV to your advantage on heroic fights.
On Feng I like to stay out of the shield about 6 yards away and pop Shadow Bulwark.
On Stone Guard I like to use it when I'm chained.
@ Feng you're intentionally taking damage, causing further stress to your healers.
@ Stone Guard you're intentionally causing murder, or wasting an entire gcd to cause very very little damage.
Sorry, but you're doing it wrong. The 25% on use effect is not as good as it sounds, being a GCD and all, as well as costing you the regular passive effect for 2 minuttes. I do wish that it would be wise to use it on occasion, but I'm pretty sure that it isn't.
Last phase @ Elegon while you're running to remove your stacks wouldn't be a complete waste I guess. But neither would a Fel flame.
I see you mentioning that your healers being oom isn't that big of a deal... That's great. If I were your raidleader I'd kick you for doing something so incredibly foolish, with the sole purpose of a minimal dps increase (I still think it's a decrease, though). Through quick math you can see that by you reflecting 140k damage over 6 seconds, you took 560k damage over 6 seconds. No strain on the healers at all. True. WHAT?! Stop raiding. Find something where selfishness is appreciated like BeJeweled or Diablo 3.
do you guys find it's worth using at all? I don't know, I think it's an incredibly weak talent, on fights I'm specing into it I'm not casting it at all, just leaving it and benefiting from the passive effect.
on Garajal heroic and Elegon heroic Mannoroth's Fury is way better anyway.
Yeah, I imagine it is. But I find the passive to be annoying in the extreme and I very rarely move about for 6 seconds. The passive on AV is a miniscule dps increase, whereas casting while moving is a dps-decrease for me personally. Simply because I'm not used to that mechanic.