There are a few problems I can see with this concept.

First off, it basically takes the build up of resources when it comes to openers and just gives everyone all of it right away. Think about starting every pull right now with full Ninki or Kenki and how much more you can do in those first burst windows than you could before. Additionally, it means any future burst windows have to be planned around server ticks of when your mana regenerates.

Additionally, there are a few skills in this game that are there to help build resources during jump phases. Think meditation on both samurai and monk. These skills become all but useless if your resources are automatically regenerating when a boss jumps with some jobs potentially getting back to full MP in longer transitions like we have in E1.

Rather than force MP to be useful, I wouldn't mind seeing melee jobs just be given a weakened form of Mana Shift as a role ability if we're doing anything at all.