I have several examples to the contrary. It was my second playthrough and I had the strategy guide and saw that I could steal a Fairy Flute from Hilgigars. That's a whole disc worth of time before you can buy it, so I thought that was a pretty good steal (get it, steal......leave me alone). So I played up until that point and I tried to steal it. I made sure Bandit was equipped. So I steal, it fails, I steal, it fails, I steal, I get a Phoenix Down, I steal, it fails, I steal, it fails, I steal, it fails, I steal, I get a Mythril Fork, I steal, it fails, I steal, it fails.....one hour and ten minutes later I give up out of sheer frustration. I don't give up on things very easily though, so I wasn't done yet. I tried this three more times, thinking that it was just a matter of luck, but received similar results as before. I knew there had to be some way to get this. So I looked around and couldn't find an answer. Then I looked in the instruction booklet and saw what the Spirit stat did. So, I looked at my equipment and made some changes for Zidane. Now, but raising Zidane's Spirit a mere 1 point, I stole the Phoenix Down and Mythril Fork on the first two tries and stole the Fairy Flute in a mere 32 mintues. 1 point of Spirit cut the time to steal a really rare steal by at least half the time (who knows how much time it actually saved me because I gave up at a little more than an hour, so I don't know how long it would have taken with a Spirit of one point less). So I'd say Spirit has quite a large effect on stealing.

And when I saw the guide for getting max stats, I played another game where I got Zidane's Spirit rather high. I was able to steal the Mythril Sword from Beatrix while in Burmecia before time ran out. I could steal the Demon Mail from Tantarian before running out of MP/gil and dieing. And I stole the Fairy Flute from Hilgigars in 18 minutes with a higher Spirit stat than before. So, by my experience, I going to have to say that Spirit does greatly improve the chance for stealing.