Holy paladins can't do it all. While we might be the best-dressed spec in the game, we occasionally need for other healers to give us a hand. The developers have done a great job giving each healer a niche for them to fill. A holy paladin must know the strengths and weaknesses of the other healing classes in order to be able to work alongside them.
While all healers are somewhat normalized by Cataclysm's introduction of the three heal model, it's each class's unique spells that give them flavor. We all start with the same base, and then our signature heals and cooldowns allow us to differentiate ourselves. Holy paladins can adjust their healing style and strategy to complement their partner healers instead of competing with them.