so... the will instead of reducing mana cost (1% per talent) will be giving 10% (per talent) mana regen while casting. this makes spirit not useless. this effectivly balances the amount (mages suffer the most since we have the lowest base mana of any class) of mana regen we lose from the paladin mana regen nerf from 1% max mana to 2% base mana.
does spirit help our dps? on short fights, not even a little. on long fights? yep, we have more effective manapool
on a side note, if we can double dip (why not) we can put points into arcane meditation (30% mana regen while casting) glyphed mage armor (50% mana regen while casting), and student of the mind (10% more spirit), that would give mages 110% mana regen while casting (lol) with 10% more spirit.
making this build
ideal to stack haste with (common, who doesn't think 110% mana regen while casting is the coolest thing ever?)
edit, those talents were changed, fire/frost mages get no benefit from spirit