I think SPR is mostly useful when you tank KQ bosses (or dungeon bosses) as most of them have a magic damage attack.
If you like to PvP alot then yeah, it'd be useful against mages too.
There aren't many regular mobs that deal magical damage. None that you would want to train with, anyway. I think most of the time, you'd need them for quests.
Though I'm not sure how useful the stat is compared to other stats. I do not play fighter.]
Edit: Forgot to mention +25 SP points also give 5% crit and more SP as well (not just mdef).
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