The reason that rogues, hunters, and warriors got a healing debuff is because they are classes that cannot heal. Healing classes have the ability to outlast their enemies sheerly for the fact that they can be brought down to low health and most likely heal up to full, while the warrior or rogue cannot and so must just try to burst down while interrupting heals. The aforementioned classes have a finite amount of lifespan in combat--they will eventually be taken down if there are no bandages or potions, whilst healing classes have an infinite time in combat, as they can heal and regenerate mana (unlike health) in battle.
As such, the healing debuff is to try to level the playing field for these classes (and work very well). Since they get less HPM with the debuff up, they cannot have, in a sense, "burst healing" to recover themselves and gain the advantage over a melee non-healing class that they whittle down.
Basically, in a sense, healing debuffs were put in as a way to overcome healer-players that fought battles of attrition.
Hunters, actually, received a healing debuff as a redesign of Aimed Shot, which no longer is the powerhouse it was before, and only good as an opener. They received it because hunters were incredibly underrepresented in Arena, as well. I'm of the mind that they didn't really need a healing buff, but it was simply a way to make them look more desirable to take in an Arena team.
Retribution paladins, equipped with both a heal spell for themselves and a healing debuff for their enemies, which tip the favors towards the paladins--which may not be a bad idea, but of course, we all want to be wary of unbalancing in the opposite direction when we balance.
I have to admit, my opinion on retribution paladins' damage capability is biased. I am of the mind that as a healer/tank hybrid, a paladin should only have the DPS capabilities to do "well enough" in PvE and PvP while providing lots of support for groups, in the form of buffs and debuffs. Inherent class effects of the Battlemaster enchant (though there is Judgment of Light), something along that vein.
And, frankly, a healing debuff is in line with my suggestion. It's simply that I don't see a healing class getting a healing debuff, which is (one of the reasons) why I protested the healing debuff on Flametongue Weapon when that was in the patch notes. In fact, I'd rather see Ret paladins having an on-attack proc that allows all party members in 40 yards take 25% more healing.