I'm not even a tank main yet I understand the value of Shirk...to the point I'm too lazy to swap it out whenever I'm doing dungeon runs or whatever (because if I'm needing something like Convalescene or that poopy parry cd, there are bigger problems). I've used Omega normal to teach fellow FC members the value of Shirk by indicating that with that extra bit of threat transferred over, I'm able to spend all of maybe 10 seconds of an entire fight with Grit up (at the start), then it's on to pound town.
Then I look at my ACT and more often than not I've been the top dps.
I like putting out the damage, no matter what role I'm playing, but c'mon...
The "buy back/refund" deal that WoW has with loot is something this game could sorely learn from. You're easily a honest misclick away from buying something you don't need whatsoever. At least in your case, you still picked up a fending piece and not, for example, a maiming helm instead of the fending helm.