As far as crafting goes, you should level all crafts evenly due to the fair amount they can help each other, and sharing the same gear. As far as its usefullness, up until endgame crafted pieces are on par with looted/rewarded items (if you make them high-quality). At endgame you can't make the BEST gear overall, but can make pretty much second best, with some pieces being situationally better than pieces with better item levels.