Spirit has nothing to do with magic defence, aside from spells that may inflict a negative status. Even then it has nothing to do with how much damage you might take from those spells initially (like Bio).

Lou, I'll test the stealing/Spirit thing in about an hour if you're still interested, right now I'm doing homework. btw...

The instruction booklet that comes with FFIX says that a higher Spirit increased that likelihood of success for stealing, Iai Strike, Shock, etc. ect. and increased the likelihood of success and longevity of status effects.
I have never seen Shock miss its target. Have you?