The way we see it, Intellect increases the power of your heals. Spirit (and other regeneration mechanics) replenishes your mana. Having Intellect also increase the size of your mana pool complicates things. It's harder to balance, and feels worse for players trying to balance their character.
While we think the healer mana model for Cataclysm is sound and ultimately accomplished what we wanted, it was still a little too difficult for a on fresh level 85 just going into dungeons, and a little too easy for raiders. Part of that comes from what improved stats do for healers: they get bigger heals (from Intellect), the ability to cast more heals (larger mana pool), and the ability to cast those heals for a longer period of time without running out of mana (as a result of regen). Along the way, the tank and the group are take more damage from tougher bosses, but also have higher damage and higher survivability from improvements to their own gear.
Please remember, the goal isnít to make healers so resource-starved that they canít heal. That isnít fun. The goal is to reward healers who limit how much overhealing they do (in other words, play smarter) for their efforts. You limit your overhealing by doing things like casting a smaller heal when a smaller heal is sufficient, or casting a slower heal when death isnít imminent, or casting a single-target heal when the group isnít all taking damage at once. Skillful healers should prosper. When players feel like increasing their skill doesnít increase their success, they tend to get bored or frustrated.