It depends on what you wish to do with your crafts. If you want to be able to craft high level items and make them high quality, then yes it is necessary to level all (or most) of them to at least 50 (some past that). It's because of the game's cross-class system: you can't really do as much with the abilities from one class than you can with abilities from five classes.

Same goes for battle classes: for each job, you will have to level the base class of the job, but also two other classes for important cross-class skills, and without them your job simply isn't nearly as powerful. It's beneficial in many ways to level all your battle and other classes on one character.