I just bought the S2 chestplate, originally for my weekend arena matches. But then I compared it to what I have been using up to now (bold), S2 is better for tanking even if I socket it with DPS gems.
5/5 S2 with the shield has 181 resilience, or 4.6% crit reduction, tons of armor and stamina, unparalleled threat generation; but obviously low avoidance, however that can be compensated with the other slots.
This seems to be the way to go for any guilds at or below ours in progression: Gruul on farm, working on Magtheridon.
It "can" work, but you still wanna maintain a nice pocket of avoidance.
Having all the effective health in the world is nice, but unless you're pushing Bear stats, you want to preserve your Shield Block charges, and the only way to do this is to have an effective avoidance amount as well.
When I first saw the Season 2 gear I thought about wearing it for tanking, and decided to do some math to see if it was worth it.
Kazeyonoma is right, the trade is a bad one.
Wearing 5/5 S2 gear over 5/5 Warrior T4 gives you the following, with everything socketed w/ SSOEs and Powerful Earthstorm Diamonds for metas:
+42 Hit Rating
-89 Block Value (109 w/ talents)
In terms of Effective Health, if your unbuffed stats are 13K Health, 13K Armour, 300 SBV, wearing full S2 gear gains you 5% more Effective Health than T4. Raid buffed, this gain will shrink.
By comparison (I did this a while ago), if you look at two T5 Warriors with access to the same gear -- one that chooses items that provide the most effective health, the other following a more typical route (5/5 Tier gear, highest ilvl items in all other slots), the first tank in this example will have 19.0% more effective health, while losing 10.1% avoidance in exchange.
19% effective health gain for 10.1% avoidance loss
5% effective health gain for 13.49% avoidance loss
More effective health is not necessarily always the best choice. As usual, it depends on how much of one thing you are gaining and how much of another you are losing. In the case of S2 gear, the exchange is not worth it. Don't get me wrong, certain items such as the shield are definitely worth getting and using, but the entire set is inferior to T4.