There was a question about back enchants for a pally healer. After looking it up and reading a bit on it I can say that it isn't a worthless enchant, but it is definitely the least effective for a pally healer.
Threat works on the principle of 100 points of damage = 100 threat (or 1:1 ratio).
Blessings, talents and other stuff can modify that so that either more or less threat is generated, usually based on the role of the group/raid member (tanks look for more threat while dps looks for less).
As far as healing goes, 100 healing = 50 points of threat. All overheal is ignored. So healing for 200 hps but 100 of it being overheal you'll only receive threat for the healing done (100 healing = 50 points of threat).
Pallies have an added bonus in that 100 healing is actually only 25 points of threat (50% of other healers).
This allows us to take talents like "Improved Righteous Fury" and put it up and still remain under other healers base-threat and take 6% less damage (woot).
100 healing = 25 threat * 1.9 = 47.5 total threat
This also means that if we are the only pally in a group or there are only holy and prot pallies around we don't need to worry about salv on pallies (although ret pallies NEED salv).
Finally, why i think the -2% threat isn't worth as much.
Assuming we are not using Righteous Fury and no other threat modifiers working for/against us:
100 healing = 25 threat - (25 * .02) = 24.5 threat
so we don't benefit nearly as much as anyone else who uses that enchantment (DPS benefits the most).
For example: 8000 heal holy light = 2000 threat - (2000 *.02) = 1960 threat