You've 6 item slots, use them wisely to balance your characters. I think if you've enough Magic Balls, it becomes powerful enough and useful in about every chapter if you have correct abilities(not 100% sure, but this is plan). Also remember spells uses Magic Power too.
In the next version you're able to use your magic balls four way:'
1) Use them as magic damage and keep them
2) Use them for some quests and stuff
3) Sell
4) Use three Greater Magic Ball to create a Zozi. One Zozi is needed to create a fourth ranked Synthesis Item. You can also find one Zozi from chapters 6, 7 and 8. At chapter 9, you can buy as much as you like, each Zozi however costs 10,000 Gold.