One Reddit comment[i] described the problem of using Mana Points in an RPG (as opposed to spell slots):

If itā€™s a game with more-or-less d20 fantasy or JRPG video game progression, then itā€™s very difficult to find a cost for an ability early in the game that feels worth the price when you have max 8 mp that doesnā€™t seem unreasonably efficient when you have 100 mp. Cure for 2 mp seems fine at level 1 when you can do it 4 times. When you can do it 50 times, itā€™s kind of a design issue.

And if you really look at them, a lot of JRPGs are actually very imbalanced with MP. Itā€™s not a problem because itā€™s a single player PvE game, but even then you might have issues with class balance.

When you switch to MMOs like WoW where that isnā€™t the case, you start to see things likeā€¦potent abilities with 1 minute, 5 minute, 10 minute cooldowns, or requiring orchestration of abilities using timing to reach maximum effectiveness, or having to balance efficiency with mana recovery rate.

And even then, most CRPGs and JRPGs have only three categories of spells: combat-only spells, healing spells, and quality-of-life travel spells. Thereā€™s seldom anything else at all. Meanwhile, magic in most TTRPGs is allowed to do any number of things outside of combat.

da_chicken has stated the problem well, but it has a number of odd assumptions. Why scale from 8 MP to 100 MP? Why not 8 to 16? Why bother looking at classes? Even in a class-based system, if you want to balance magic vs the sword, you could just cross-measure those two things.

The assumptions seem to come from computer-games, but the final paragraph illustrates why these assumptions donā€™t carry over to TTRPGs well.

BIND has always used MP, but avoided these problems. The ā€˜balanceā€™ probably comes from a single, early decision: low-level spells are the most efficient.

First level spells (which cost 1 MP) do ā€˜3 somethingā€™; it could be 3 Damage, or remove 3 AP from someone, or give 3 Fate Points (FP). Second level spells (which cost 2 MP) do ā€˜4 somethingā€™, and so on, with +1 per level. If you want to inflict Damage, then itā€™s better to spend 2 MP on two spells and deal 6 Damage, than it is to spend 2 MP on one spell, and deal 4 Damage.

However, the higher-level spells have more range, or target many people at once, or do something else fancy. So higher-level spells are great if you need a blast of fire which forms a demonic hand, then reaches into a window, and sizzles just the right person; or if you can spread fire over four enemies without hitting any allies.

Of course higher-level casters can withstand this inefficient casting, since they have more MP to spend.

[i]