Its because healers don't dish out any real damage in healer stance. The armor is stance specific, so the stats add to what the stance does best. If you're not buying healer armor you will have lore trouble with healing in raids and harder alerts. Not to say you can't mix in a dps armor, but you should have mostly healer armor if you want to contribute to raids, where these stats boost make the most difference. I like to solo sometimes, so I have a spar dps gear to wear when I don't group and then just use my tier gear for groups where I'll be healing.
If you buy the damage piece then you have made yourself a dps and should never be in the healer role. Unless your innate healing stats are so good you can make up for a couple pieces of damage armor. Once you get to level 30 you need to focus more on your specific role whether you choose it to be a healer or dps.