I'm leveling Conjurer, Thaumaturge, Weaving, Carpentry, Leatherworking, Alchemy, Goldsmithing, Botany, and Mining and I use crafting leves to fill in gaps where items are too expensive to make or require materials that I cannot find. That's 9 classes and even with 9 classes sometimes I still don't use all 16 leve slots. I think it's better to pick and choose which leves benefit you at the time instead of just grabbing everything they give you and running out to do them. Look for leves with rewards/marks, leves that will help you fill in on crafts when you're low on materials or whatnot. For Weaving I found hundreds of Moko Grass from Botany and I used it to level my Weaving to like rank 15 by turning the grass into yarn, yarn into cloths, cloths into cowls which I then sold for good gil. Now since I'm at a point in weaving where the hempen stuff is too low and the cotton stuff is too high, I rely on weaving leves to level. It's all about managing what you're given with what room you have, you can just grab everything and go out and spam leves but you're not going to get as much out of them as someone who picks the leves that are most rewarding and taking that extra time to maximize on what's available.