There is literally no benefit at all to leveling crafting and gathering on a separate character, aside from more inventory space. You may feel like guildleves are limited for now, but once you miss a day and especially when your XP options start opening up to non-leves you will suddenly find yourself at the 99 cap and unable to use them all up unless you really tried. Unlike FF11 you aren't limited to leveling only one thing either, you can level as many craft and gathering classes as you want.

As was mentioned, being able to repair or meld your own gear is pretty damn useful and especially with Hamlet defense being the beginning of mixed-discipline content that has rewards for all of them, you really do want to do stuff on one character.