It depends on how you are talenting. Holy Priest depends on spirit because of Meditation & Spiritual Guidance which are used for end game healing.
Base regeneration rate + MP5 * (2/5) = Regeneration rate while casting (remember, Mana ticks every 2 seconds!)
Spirit increases Priest Mana regeneration while not casting and, if you have taken Spiritual Guidance in the Holy tree, it also converts up to 25% of your total spirit to Spell Power.
Whenever Mana is spent for any reason at all, your Mana regeneration gets halted for 5 seconds. During these 5 seconds, any regeneration based on Spirit does not happen, unless you have certain talents to partially counteract this. As a Priest, you can spec into Meditation. Note that the rule of needing to have Mana spent means that you can safely cancel casts before they finish, as that will not incur the 5-second rule.
Stat Priority for Healing Priest in order of importance.
Healing Power
Mana per 5 seconds (Mp5)
Spirit
Intellect
Spell Crit
Stamina
Yes, Mp5 is desireable but Spirit, with the above mentioned talents is a very close 2nd and almost even. I thought I read back in the day that it also increases our mana pool but I’m not seeing it mentioned on icy-Veins. I know Intellect definately does.
I’m not sure why so many Mages & Warlocks get so heated by the thought of yeilding spirit drops to Priests. I mean, how many Mages and Warlocks have you seen that actually stop dpsing for 5 seconds to rebuild what amounts to 1 mana per 5 seconds?
I just tried to refer a Mage who needed on my preraid BiS and won, to Icy-veins and he took it to a whole new level. I see Warlocks who are also amamant that they should be needing on Spirit gear that a priest is needing on so I’ll just leave this here:
Stat Priority for Warlocks in order of importance:
Spell Power (or +Shadow Damage)
Spell Crit Chance
Hit Chance until 16%
Spell Penetration
Intellect
Stamina
Spirit.
Spirit increases your out of combat health and Mana regeneration, but you can get the Mana regeneration in combat by following the 5-second rule. The 5-second rule is that you will begin regenerating Mana when you do not spend any Mana for 5 seconds . This is pretty useless for Warlocks.
From Icy-Veins:
For Mages, Spirit provides Mana regeneration, mostly when outside of combat, which is very useful while leveling but not so much for maximum level content.
Stat Priority in order of importance:
Spell Hit Rating (you will typically need 10% to cap)
Spell Critical Strike (150% damage chance on spell hit);
Spell Damage.
Intellect (grants spell critical strike and increases your maximum Mana).
Other Stats:
Stamina (more health per point);
Resistance (strongly reduces Magic damage taken of a specific element);
Mana every 5 seconds (for Mana regeneration while you are casting);
Spirit (health regeneration outside combat)
Agility (dodge);
Armor (less physical damage taken).