Probably because godhand is holy, and I was speaking only in pure damage potential. Since Mirte has friendly fire, I didn't really like having her in my party. Her final magics are the ones you unlock by investing, they are pretty powerful, don't have a miscast chance, and there are several of them so you can cast others things while on cooldown.Maybe I have just been lucky/unlucky with damage type vs. Enemy or have not gotten Mirtes ultimate magic. Because Marisels Godhand usually wipes all enemies in ways the Spells Mirte have does not even though Marisel only got around 800 magic stat and Mirte around 1.200.
To my playstyle I would still rate Marisel more useful in dungeons since I can turn off my brain in regards of party health and MP management . But I agree it is easy to manage MP and health without her.
Anyway, the only caster I liked to have in my party was lucerola, the drains have a low cooldown and can be spammed, and she is also perfect for mana management. I find melees easier to manage, they are more tanky and their damage is more reliable.