What you see there is a description of the Devastate talent in 2.1. It states, "An instant weapon attack with both weapons that causes 50% of weapon damage plus 15 for each application of Sunder Armor on the target."
Now, I played around on the test server a bit, and Devastate was hitting for the same as it does on live when you have a Sword / Shield out. I didn't get to do any real testing yet with two weapons...but I'm curious why our 41 point talent for Protection just keeps leaning more and more towards dual wield. It seems they want to give us a talent for farming rather than any sort of passive threat scaling upgrade.
You get 2 attacks while dual wielding, much like Mutilate. Right now the offhand damage modifier is only applied to the weapon damage. You still get the full 175 damage for 5x SA, even with your offhand. With Mallet of the Tides and Blazeguard, I was hitting around 300+200 in full tanking gear.
Devastate is a decent replacement for SA while tanking once you reach 5 stacks. A bit more threat than unimproved SA, a bit more raid damage.
The second purpose is protection warrior DPS. Sometimes you just can't get around it because of encounter design. With the recent change, it will be even better than before.
It certainly ups the DPS of prot warriors that aren't tanking (something feral druids have a huge advantage in atm--that is, being useful if they're not tanking) and perhaps gives people that aren't tanking an easier time trying to stay up on the agro list given that they aren't getting all the rage from being hit.
The change doesn't affect someone who always tanks, except for those few encounters where everyone DPS'es, like Aran. But it is very good for offtank DPS and probably for staying second in aggro, if that is ever required.
Edit: "The change to Devastate in 2.1 is being reverted and will not be a change for the patch once it goes live. We've done some additional damage testing and the output is in far excess of what we had thought it would provide"
There’s some indication that Devastate is now in a state where the off-hand gets no bonus from Sunder Armor, but both hands still get an attack. (I have not yet been able to get on the test server to corroborate this.) Presumably, this would be a tweak from the devs to see if the dual-strike Devastate is workable at all.