In theory, it's a brilliant idea but when it comes to practical, unless the game employs a lot of strategy and stuff, this "idea" seems somewhat complicated to program. The game needs to "remember" the mana regeneration rate for the different walkpoints the player passes through. Plus, you've got to ask yourself: what benefits or disadvantages would implementing this bring forth?
Unless it's limited to only certain key areas, I think it's better laid off to rest.
