Mana access during a battle.

[Ryastar]

Mana potions are only usable outside combat, and their purpose is to give you some quick mana if you are low before fighting something. If you don't care about time, yes, they aren't incredibly useful.

And nitelite, thanks to the wonderful pause button, you pretty much always have a chance to quaff a potion, no matter how close the enemy is. There are many situations where there is no possibility of running away.