There is a thread talking about materia's on JP side and they talk about how some materias should be melded onto other parts of the gear or how elemental, stat resistant materia's are crap. Anyways Triairy responded regarding the Manathirst (MP+ materia).

Quote Originally Posted by Triairy View Post
こんばんは。
Good evening,

智者のマテリアの効果が低いというフィードバックを頂いていますが、自然回復 するMP量は最大で自身のMPMAXの7.5%になっており、これに智者のマテリアの効果でMPMAX+100が付いている場合、さらに3秒につきMP7 の回復量上昇が得られることになります。そのため、智者のマテリアの数値は現状では適切な数値だと考えています。
We received feedback saying the Manathirst materia's effect is low but with auto heal the maximum it will heal will be 7.5% of own MAX MP. So if you have MP Max+100 with materia's that means you will gain 7MP more every 3 seconds. For that reason we believe the Manathirst materia's value is optimal.

ただし、今後のフィードバックや他のマテリアとのバランス調整次第では性能の見直しを行う可能性もありますので、引き続きフィードバックを頂ければと思います。
However, with feedbacks depending on balance with other materia's it may be possible it may be changed so please continue to leave feedbacks
wiwi posted on that thread right after saying
Quote Originally Posted by wiwi View Post
ファイアの消費が294(LV50）なので、智者の効果でMPが100増えれば126秒＝つまり2分休憩すれば一発ファイア分お得になる計算ですね。
Fire MP cost at Lv50 is 294 so with Manathirst effect getting 100 MP will mean 126 seconds = so with that calculation you rest 2 minutes you get to cast 1 free Fire spell.

むむう・・・適切・・・？
hmmm....optimal...?