If I'm lucky with my guess that each BM would be stacking stuff independently, that means there might be someone who will not be covered by shields and, therefore, will likely die to that Big Boom. Also, it is very rare for multiple tanks to have high vengeance simultaneously. Without Vengeance Guard will be absorbing little to nothing. I'm not saying it will be completely useless... It just requires quite a rare coincidence of fight gimmicks like high vengeance+high damage opportunities immediately followed by raid-wide AoE. And that still leaves a chance that someone particularly squishy will not be covered by any shields. It's randomness of such tactic that makes it highly unreliable => undesirable.
Thats why I said double BM, not just one. As long as there isnt constant AoE damage going on (like H will), you can shield the whole raid. Think Blade Lord, there isnt much aoe damage going on before Unseen Strike so you could get statue shield on everyone and just eat it without cooldowns. Or on Amber Shaper in p2, if most slimes are dead in later p2 you could shield the raid and eat amber explosion (for whatever reason). Just examples what you could do that might allow some weirdness in t15.
Preventing constant AoE damage like H will is nice, but wasn't really what I was going for.
Problem with Xuen isnt that taunting mobs is necessarily "bad" (unless we are talking spirit kings, thats just stupid), its that more often than not it dies because it taunts adds. Also since it acts as a guardian rather than pet healers have a hard time getting around to heal it.
Yeah, t h a t part of Xuen is smth I don't like as well. Fella seems to last long enough, though...