Just as rogues and hunters can get more haste to increase their energy/focus regen, healers will be fully capable to stack spirit to new extremes (for pure regen, no throughput gain like haste for DPS). Considering the number of gems/enchants/flasks that can be swapped between intellect and spirit, there is quite a large area for healers to balance their spirit and intellect levels in a given gear set. The percentage by which a healer could instantly boost their mana regen through swapping gems/enchants/spirit seems enormous. Given item inflation in later tiers following this change, I can only imagine mana bars looking like a haste stacked MM Hunter's focus bar with Hungerer+Matrix procs and RF running simultaneously.
If the quantity of stats are to remain the same on gear, there would need to be a fairly aggressive DR on spirit regen to prevent the exact problem that is supposedly being fixed by this proposed change. The "Squish" has already been proposed. I believe that might be a better concept to attempt first before trying to balance that with an entirely new locked resource for casters.
Changing to a purely spirit based regen system would also likely put even more value in acquiring items with functions like Shard of Woe, Jaws of Defeat, and Alchemists' Stones, which could permit more stats to be moved back into throughput.