Yes u have a point here, but... its about a mixture of figures that combine together make a whole.
when a tank start karazhan he must have at least 18% dodge, 18% parry and 20-21% block chance rating... so a amount of 56% avoidance chance.
about the gruul and 25 mans raids... for alot of times i didnt get healed bcs of not my problem reasones

and my avoidance rating saved me and the group. u cant be healed all time in gruul (ppl needs to run, move etc). anyway u know what im talking about.
about to much avoidance = no rage, u are right but that is only for kara. gruul hit me like more then 40% but less then 50%... the real problem is to have heal after the shatter comes (thats the critical point of the fight.. with all my hp i cant survive with no heals)
now... i rly dont think that those gems will change my chance to survive in a positive way (800-1k hp?). if i dont have the right amount of avoidanve ill get hit more often ant therefore much hard to be healed. i did change my gems and made a mixture of def+sta. and im shure all of us have stuck in the head the words "im out of mana".
blizz didnt make the gem color thing just by mistake or for fun. they have a team that calculates the wow mechanics (so from those gems i have with 2% more dodge and 2% more parry, like 2% more miss chance and 2% more block = so ~ 8% avoidance, maybe more). and im shure alot of tanks dont have a clue about 102,4% combat table, so maybe thats why they use only blue gems allways (its the easy way to fill a socket whitout thinking... they do so bcs they saw on another warrior)
from my opinion its the same thing.
if i have alot of avoidance i dont get hit much and healers can focus on event tactic so they dont die.
with more sta i will get the hits for shure so i need to be healed all time by all means.
atm i have 18,300 hp buffed, so 19k hp will not make the diference for shure.
our gruul problem is that after the shutter i dont get healed, and there allways must be someone near, me so i get at least 5k dmg from the shatter. after 5k shatter dmg ill get hit for another 6-7k after lets say 10 grows, so if i dont get at least 2-3k heal ill die for shure (i cant drink after each shatter).
another very important thing is the armor... more armor i get with more less ill get hit... 1 point of armor = 1 point of sta.
a warrior with no avoidanve its not a tank, hes a tombstone. i saw a kara / gruul build warriors with 38% avoidance thats just lame.
anyway out there are alot of guides and documentation about wow mechanics and playing style. yes after kara raid a tank needs alot of sta... but u cant just erase avoidance and get 1-1,3k sta instead

just bcs this, there exist mixture gems with def+sta or agi+sta or str+sta so the avoidance can make the diference in the first place.
now... i think after a tank get fully geared from kara hes rdy for maulgar / gruul encounter. from nightbane drops, chest / shield, i will have with 41 sta more so 410 hp... armor, block, def and so on.
but that is not only for tanks... is for all classes. if u get all kara gear for ur class gruul will be much easy for shure.