If we look at the resource systems of WoW over its lifetime, it's clear that Blizzard has been trying to diversify the mechanics. We've seen warriors' rage system constantly tweaked, rogue energy regeneration altered in every expansion, and hunters returning to their old focus resource from back in the original beta. The inception of death knights also introduced not one, but two resource systems to the game. Warlocks are even getting a new soul shard system in Cataclysm that will give them a brand new resource bar.
Paladins of all specs have been having problems with their ability usage. Retribution paladins could use a castrandom macro and do nearly full DPS, protection paladins have an incredibly static "969" rotation, and holy paladins have been stuck using nothing but Holy Light since Naxxramas. We needed something to help us break the cycle of repetitive ability usage. The dev team looked at ways to give our class a little more flavor, and came up with the concept of holy power.