#1) Tanks have better defensive cool-downs. Do you see any Marauders tanking bosses?
Answer: PVE enviroments consist of constant large attacks by boss mobs. In a raid, the tank takes so much damage that his defensive statistics make him a better choice than a marauder. Obviously a tank with better defensive stats is a better choice to survive the millions of incoming damage tanks are healed through in a raid night. A PVP environment is completely different; consisting of short bursty encounters. It is these short bursty encounters that make Undying Rage/GBTF the most powerful Cool-down in PVP.
#2) Undying Rage/GBTF really isn't that powerful because you can be stunned for the duration, and Force Shroud is better.
Correction Juggernuats have beter defensive cooldowns, Vanguard tanks only get a 15 second 25% mitigation buff that has a 2 min cd, adrenaline rush hardly counts has a defensive - 15% max health over 10 seconds its not even instant.
Number two only applies if you stuns are off cooldown and the maur does not have resolve immunity. I only have two stuns one for 4 sec with 2 min cd and the other 2 sec with 45 sec cd, both cost 1 ammo or 1/12 of my resouce