As you can see, the enchant is always the best except if you rate both effective health and threat double as important as mitigation. No good paladin tank with gear on T8 niveau has trouble with threat, not on normal bosses and not in special cases like Malygos and Hodir hard-mode, so no good paladin tank will rate threat over mitigation. Not sure about other tanking classes.
Just found out about this enchant on a new post from tankingtips.com and I must say I was annoyed that I did not know about it. I tend to pride myself on perfecting my toon and had I known, I would have taken this enchant a LONG time ago.
I currently use a 5/10/56 build which creates far less threat than some other tanking builds but I still never have an issue with threat. So for me there is no question that this is the best enchant available. My back of a napkin calculations lead me to think it should increase my effective health even more than the Heavy Borean Armor Kit that already has a 15 score.
I realize that this is a BC enchant but even if you look at the ilvl budget (240/14 = 17.1) it is somewhere in between 15 expertise and 20 threat. But for those tanks who know their rotation and use heroic strike, threat is not an issue making this enchant the hands down favorite.
I've used this site before and had it mentioned this enchant as a possible alternative I wouldn't have hesitated in taking it. But without that knowledge I've gone through most of WOTLK without it. Please bump it up to 15 points for no other reason than to let other tanks know it's there.