Yeah, the GBA version gives you waaaay too much MP and LVs to make you use anything but the most powerful spells. That version is the easy type by default with no difficulty option. :\
However I am a fan of the newer MP system. I was playing through the Mod_of_Balance hack and it is VERY close to the original. Obviously some things I don't like so much but others were done very well. My Black mage uses the most powerful spells only once or twice per Inn visit, and still casts fire/ice/sleep at LV 14 often. He did a pretty good job with that I have to say.
..Ah right, spells. ..ooh a piece of candy. Ooh a piece of ca...
[edit] Actually wait a minute. There's no data for it in the ROM, which means it is done using some math. ...and that is confusing.