There is certainly an enormous benefit from leveling multiple crafts. Not only cross class skills that are extremely beneficial, but also the ability to crafts items you need. You'll often find that you need an item from another craft in order to finish a synth. Maybe you need a silver ingot from GSM to finish a WVR synth, or an Iron Ingot from BSM to finish a CRP synth. You just never know. The ability to make the items you need and not rely on the markets can be highly beneficial. Plus you'll begin to be able to repair your own gear, which is also nice. Lately I've started using my GC seals to buy Dark Matter to repair my own gear which allows me save gil I would have spent on repairs. Which was, until recently, my only real expense in crafting and harvesting. Still need some more levels on WVR, GSM and LTW, but I'm starting to get to the point where I need to spend little gil on repairs.

There are a great many benefits to leveling multiple crafts.