I think I found out what's going on.
Intellect is better if it's passive, i.e. if it's always there. The mana pool will be bigger, the heals will be bigger, the crit will be bigger. IF you sit and do nothing, mana will be saved.
If you proc Intellect randomly, you got a huge problem. It may proc when you are full mana and there's no much need for "spam" yet. You will waste the proc.
It may proc when you are in the middle of the fight with half mana but it won't save its mana when you don't do much and then the mana bar drops down.
So, I think it's obvious, Spirit should be selected on random procs and intellect on passive (of course, with some reasonable parity, same ilevel, not 10 Spirit vs 200 intellect etc.)
One could say Spirit could proc in the beginning, but even in the very beginning most healers will have about 10% down so it's even there a save.