Every class was designed with a specific flavor in mind. If you read Blizzard's official descriptions of the classes, you'll see that skills and abilities were not assigned at random. These paradigms of thinking for each class pulled from fantasy archetypes and characters from Warcraft's rich lore. Each class had a purpose, and those purposes were what made the game diverse. With 40 people in a raid, you could easily assume that every one of these crucial roles was filled. Unfortunately, that doesn't carry over to today's raiding scene. With the seemingly constant shrinkage of the de facto raid size from 40, to 25, to 10, it's become more and more difficult for the developers to ensure that we'll have all of the tools and abilities available in the game.
Blizzard's faced with the tough challenge of trying to ensure that each class stays unique, but also allowing for enough overlap that you're not forced to raid with a perfect mix. Bloodlust has always been the posterchild for this war between uniqueness and homogenization. Shamans have claimed that Bloodlust is their right alone, but the developers decided to give the ability to mages as well. Discipline priests, the sleeper healers of Wrath that went from useless bubblers to raid-shielding gods, were next in Blizzard's sights. Luckily for us, the devs chose paladins to be the recipients of this socialist disbursement.