@Ghostly
First off, 75% of the raid bosses are immune to taunt to begin with...
And secondly, both Challenging Shout and Taunt can and do miss since they are actually considered spells and thus require +Spell Hit to work properly which is why Warrior Tier 3 Dreadnaught armour has the following set bonus:
4 pieces: Improves your chance to hit with Taunt and Challenging Shout by 5%.
http://db.vanillagaming.org/?itemset=523 And warriors would use this trinket in combination with the T3 Dreadnaught set bonus in order to 100% make sure that taunt would work in the Four Horsemen fight:
Nat Pagle's Broken Reel
Binds when picked up
Unique
Trinket
Requires Level 60
Use: Increases the chance to hit with spells by 10% for 15 sec. (cooldown 1.25 min)
By valkyr on 11/25/2006 (Patch 1.12.2)
Makes Four Horsemen a joke. 4 piece Dreadnaught takes spell resist chance down to 11%, so with this you have a 1% chance to resist taunt.
The cooldown is only 1.25 minutes, so by the time the tank has to get to the next station, its cooldown is up. The fight is certainly possible to do without it, just makes learning/executing the transitions a whole hell of a lot easier.
http://db.vanillagaming.org/?item=19947/thread