Bear with me for a moment while I try to explain what the gaps are.
Here's how I classify the major roles in the game, most of them are dichotomies: magical/physical, melee/range, healing/damage, tanking, buffing/debuffing.
For example, hunters are ranged, physical, damage. They also have ranged and melee physical debuffs.
Priests are ranged, magical, damage and healing.
Now where are the gaps? There are combinations which currently do not exist in-game. There is no melee magical damage and no melee magical healing (though there is physical: bandages). Ranged tanking is sometimes filled by specialized cases of kiting or mage-tanks on HKM.
Holy can help with these gaps. Buffing its damage and adding significant healing components to damage would push it into melee and create a melee magical damage and a melee magical healing spec.
Guest post: The mind of a tank
8 hours ago