My reasoning isn't to nerf Holy Paladins as healers. I love all healers equally. Rather, my idea here is to incorporate a class design that's been picking up steam since the end of Burning Crusade. Back before patch 3.0 dropped, two Paladin specs wore spell plate, Holy and Protection. Since that was two specs out of a possible six plate wearing talent specializations (2/6 = 1/3 = roughly 33 percent, I'm not doing a repeating) that meant that there was a fairly sizable minority that wanted the stuff.
Flash forward to Wrath of the Lich King. There are now three plate wearing classes. Two of them (six total talent specs) have absolutely no use for spell plate. The one class that does use spell plate? Well, the Protection tree was revamped, so tanking Paladins don't wear it. Retribution has been giving spell plate the stink eye forever. So we now have 1 out of 9 possible specs wearing the stuff, and yet it drops everywhere. There's no reason spell plate needs to exist, much less be a protected species by Blizzard when they went out of their way to do away with tanking leather, for example.