Caydien from Blizzard took the time to give a little more background on the changes for the 1.9 update. Be sure to read the whole post.
You've asked for the philosophy and reasoning behind the Paladin changes; after speaking with the developers, I've returned with some notes on why the talent trees were changed.
Now, each tree has an overall theme for what it's meant to accomplish: Protection is the defensive tree, upping survivability and defense; Retribution is the damage tree, increasing the Paladin's capacity to dish out DPS; and Holy is the "holy magic" tree, increasing both healing and damage spells. Now, these are the overall ideas behind the trees, but they aren't completely specialized towards these goals -- creating a tree that's completely specialized in one direction causes players to be pigeonholed even further, which is something the designers want to avoid. As such, there are some diverse elements in each tree, such as Blessing of Kings in the Retribution tree. (A similar effect is seen in the Druid talent trees, with both beneficial feral and casting abilities spread throughout.) With a hybrid-like class such as the Paladin, it's important to promote that diversity rather than shun it for over-specialization.
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