So if Parsimony halves MP cost.....?????

[Eldaena]

Basically, you get a return from using parsimony. Say a spell costs 500 MP, you need 500 MP to cast it (even though it costs half of that) and in return you get (random number of) MP back. This is significant from a standpoint that if you had less than 500 MP you wouldn't be able to cast it. Why is it this way? I'm going to say because there isn't much you can do with 250 MP normally, let alone cast a spell as powerful as one that would cost 500 MP. Perhaps if you think of it as a roleplaying aspect, you need to have enough MP reserved to cast something powerful enough to get some magic points in return.

Parsimony

Noun:
Extreme unwillingness to spend money or use resources.

Synonyms:
miserliness - thrift - stinginess - frugality - avarice

So, perhaps you have to have MP to make MP? lol

Edit: It is also factors such as when the spell can be interrupted if you suddenly lose enough mp that it will not allow you to cast the spell (like Tailturner in the Moogle fight), or if there is some kind of effect in the future such as: "you cannot gain MP" or "the MP you gain is increased for a duration."