Actually, I believe I read a quote from a dev somewhere stating that casters don't have unlimited sp, but that the encounter doesn't last long enough for them to go through it all. Considering that they get to blow all their sp at one time, while we have to measure it out for an extended time. If I was less lazy, I'd try to find it, but I'm lazy today.
That having been said, I don't think it's entirely accurate, since I've seen enemy casters go for a REALLY long time, spamming their spells in a futile attempt to get me

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