You're missing the context here. We are talking about using shield block for extra mitigation at the expense of threat generation. It's not free. I had initially said it's a waste, which may be a bit strong. I'd still tend to spend the rage on threat instead of blocks - but it will depend on the fight I suppose.
Edit: And it's not a static amount. You can still be missed, dodge, or parry the attack if shield block is up. And the healers are still healing.
Ultimately, it's a choice that can't be wrong whichever you choose.
Sorry, I thought you were asking why we treat SB/SBV differently than avoidance.
And I do agree you should use SB conditionally (I never use it on trash, and always use it on bosses), but I respectfully disagree with your views on the statics of SB. IMO, not only is it static, it gives your healers more flexability. Your avoidance percentages don't change so healers will be seeing the same amount of overheals from that. However, SB normalizes all incoming physical damage, which can be adjusted for, especially on non-crushing mobs. Personally, I try to avoid avoidance and keep SB up purely for the reason of keeping damage predictable.
I'd think shield block is still good on bosses that dont crush.
In BT gear your block value is probably more like 800 than 500 (assuming youre smart and stack it). Thats like a potential 1600 less dmg taken by using shield block vs not using it, which might save you. Its like having 1600 extra hp.
Whether its worth the rage? I'd think it is on a boss that hits that hard. You'll have plenty of rage.
"That's true, I suppose. I could argue that armor reduces rage generation too... but I won't, because it wouldn't hold up."
Armor does reduce rage generation. This is why when you tank stuff like regular 5 mans that youre super overgeared for you have rage problems, and need to put on some dps gear.
But its rage reduction is very mild, and consistent. Going from getting 30 rage, 30 rage, 30 rage to 28, 28, 28 is ok. But high avoidance gives more streaks of no rage, which hurt your threat generation a lot more.
There are many benefits of the 'stack armor/stam/block value' strategy, but generally what it does is:
Maximize effective health, thus givign you great survivability on boss fights, and:
Provide strong threat generation, since you have a constant influx of rage due to low avoidance (dont get no rage streaks), and because your shield slams are so powerful.
Avoidance prevents more damage than armor or block value (for the same amount of itermization points), but it does it very inconsistently. It thus hurts your rage more, while not improving your survivability in worst case situations when you get hit several times in a row. It also sucks when you get stunned, because it doesnt work. High Avoidance/Low stam tanks get destroyed by stuff that stuns then.
You have infinite rage on Archimonde for sure. In like 100 tries I've only found myself rage starved only twice and it was because of a very very luck with dodge/miss/parry. Shield block helps a little and on this particular fight I recomend using it.
Shield Block against non-crushers should never be used at the expense of threat, but these mobs already give you infinite rage so I don't mind use it every time its up.
In summary, TBC raiding is easy. 9/10 encounters can be summarized with 1 phrase. Stay out of the fucking fire. Panda Cub with a Gnome pet!