1).
Prot warriors don't get free damage mitigation like 15/20% from ardent defender/will of the necropolis, they don't get a billion hp like bear tanks, they need to cap hit + expertise to get good dps/threat/aggro, they don't get to stack stamina/armor only like the other classes while still easily outputting more threat than a full bis fwar like a dk tank.
Prot Warriors are simply bad tanks for RS25HC/ICC25HC and 25 man raids in general, BUT THEY ARE STILL A TANK!!!! BAD TANK > NO TANK!
Prot Warriors are better for 10 mans as damage values are lower and thus blocking will mitigate a higher portion of damage (still not great).
Basic Math Understanding for block:
*numbers pulled out of *** for concept understanding*
Warrior blocks 5k damage per hit, 10k with critical block
10 man boss = 15k damage per hit, 5k blocked = 1/3rd damage blocked
25 man hc boss = 45k damage per hit, 5k blocked = 1/9th damage blocked
25m hc boss vs prot pala tank = 45k damage hit, 20% mitigated (ardent defender) = 1/5th damage blocked = total damage 36k - paladin block
.