Starting with Cataclysm, Paladins have a new resource called Holy Power. Spells that use this resource are called Holy Power spenders.

Why track it

Because spenders scale with this resource (the more Holy Power, the more powerful the spell is) and because Holy Power is capped at 3 charges, it might be interesting to glow buttons when the Paladin has 3 charges of Holy Power. This gives important information:

• spenders have reached their maximum efficiency
• casting a spell that grants another charge would waste a charge (because overcapped)

Word of Glory

There is one spender that all specs can use: Word of Glory. Because acquiring dual spec is easier than ever, options should let the player configure if this spell will glow, based on player specialization.

Technically speaking, a similar specialization-based option was done to Warlock's Drain Soul #126 which glows to either for Affliction locks or for all locks. There could be an option for Word of Glory to let the player decide whether the button glows for Holy paladins, Protection, Retribution, or any combination of those (6 possibilities in total).