Disc favors more spellpower because it amplifies shield and healing coefficients in a big way than what spirit does. Spirit has a minor role because there would be few talents they would add from the majority Disc or the minor Holy tree tiers.
Holy however needs to last their mana for the raid and whether you can afford it or not, you're balancing SP and Mana Pool for a "healthy" amount of MP5 (and to a lesser extent, spirit) during casting because of your reactive group heal priority, so to speak. Reason I say this is based on PoV, Holy can heal effectively a raid without difficulty if it can handle the amount of time a boss fight takes. But the arsenal is fairly costly and there has to be measures to help soften that blow. Mp5 and Spirit then comes in.
Healing is the source of life. Without it, you're left with death.