The difference in MND seemed to be too minimal to care about.
However, if fights ever drag out long enough, putting points into PIE seems like it can be useful at endgame. (Not for simply having a larger pool, I dont even look at that as a reason)
The sole reason being In Combat MP Refresh.
You regen 2% of your max MP every tick (3 seconds) during combat.
So if combat drags out long enough, and u have enough MP, u can see a reasonable amount of mp coming back to u every tick.
But until I can compare endgame stats to eachother, and know how long fights drag out (if at all) I cant really make a good comparison of the 2 stats.