The locations for the mission stones was vital to me knowing where to go, and probably saved me a lot of traveling around aimlessly. It also has some interesting bits for things such as upgrading, item finding (on some of the less linear maps), which abilities each character gets in each role and (for me) the best bit was the extras which have background information for the overall story and some nice artwork. In fact, there are nice pictures throughout, if you're wanting some aesthetic value. They give advice for good leveling areas, too.

Basically it's not a must-have, obviously enough - it would be silly if you couldn't beat the game without the guide - but it does help you get though it more efficiently.