...So, in short, you want to give shugenjas a "mana pool", and give spells a "mana cost". And give extra work to both players and GMs (yes, because checking if the math is right is necessary). For a benefit of.... turning the game similar to a MMORPG, it would seem?
And tracking 5 resource pools (one for each ring) is easier than tracking only one? Tracking a "mana pool" is no harder than tracking a hit point or wound pool.
Also, one of the biggest benefits of "mana pool" systems over "spell slot" systems is the fact that they can be much more granular in the resource costs for various effects. This allows for various effects to be better balanced.
Edited by Ultimatecalibur