Paladins are balanced around the free healing that they get from using Holy Power heals effectively. The free healing means that Paladins don't spend/don't need as many regen mechanics as other healers.
Wrong, I've played every healing class in the game at some point during MoP and Holy Paladins are by far the most mana hungry of the lot, my Disc Priest can do fine with just 8k Spirit and my Monk never needed more than 7K.
Seal of Insight + auto-attacks will regenerate a significant amount of mana - something no other class can do.
Using divine plea smartly along with seal of insight will allow most good holy paladins to play with less than 12k spirit, where a resto druid may want 20k in hc SoO gear.
Also Wrong, I've talked to quite a few Holy Paladins and my Paladin has 12K Spirit and it is no where near enough Regen to not go out of Mana even when using Divine Plea on Cooldown with trinkets and other stuff.
Also this is one of many Paladins I have found with considerably more than 12K Spirit