Invulnerable Mail

iirc, this item originally did NOT have the "can only occur once every 30 seconds" restriction, and thus it provided quite a large amount of damage avoidance against fast-attacking foes, such as dual-wielding rogues. So it was very popular for PvP. But then the nerf bat swung. And I don't think the (excessive) mats were reduced.

Could someone explain this item? Warriors wouldn't wear it, and I've never seen a Shaman or Hunter tank, so... why?

From reading thottbot.com, this was once a very popular item among shamans and warriors for two reasons. First, the proc rate is set - it happens often and is essentially a pally bubble for 10 seconds. Second, this breaks when attacking but leaves you able to heal. So a warrior could bandage and a shaman could cast a quick set of healing waves. But in reality, this dropped for me out of a eternium chest in zanger. There is no point to this item now that the mats for it vastly outweigh the ease of getting something better for any class that could potentially where this. Vendor trash, sadly.