Trying to level a crafting class at the same time as a combat class is a great big headache. There is so much to do, it winds up coming to a grinding halt. But do level crafting classes. Levequests are a good, quick, way to level a crafting class if you've been saving them up forever (like I do, pretty much solely so that I have leves available when I need company seals or the extra push to level X). Just don't expect to keep your crafting level up to scratch with your main class every step of the way. It's nigh on impossible unless you start committing more and more time to the crafter.