Some really quick and dirty numbers (your mileage may vary).
[Illidari Runeshield]
[Kaz'rogal's Hardened Heart]
If we say that white + seal damage makes up 35% of your threat, and you're averaging around 900 TPS, that means:
900 x 0.35 = 315 white+seal TPS
315 x 0.0133 = ~4 extra white+seal TPS from 21 hit rating
34 x 0.5 = 17 TPS from spelldamage on Illidari Runeshield
17 - 4 = 13 TPS difference
Looking at defensive stats, the KHH offers an extra:
28 defense rating = 11.84 = ~1.42% raw avoidance
13 base stam with +15 stam gem x 1.1 x 1.1 x 1.06 = ~16 extra stam with Kings and talents
So, ask your warrior if he'd be willing to give up 160 hp and 1.42% avoidance for 13 threat per second. If his answer is no, then he should have no problems with you picking up KHH.
EDIT: This of course assumes there are no other issues to take in mind. For example, if you're a secondary backup OT and he's the full time MT, it's probably in the best interests of the raid to pass the shield to him. This has nothing to do with your class, though.