I have 3 alts ( 4 characters) and they're all different races so I basically have "free" fantasias for when I feel like playing a certain race. I also don't feel the need to unlock everything in my alts. I've done the msq and nier raid on all of them and I just login into them once a week for my weekly nier item and that's about it. I'll login to visit friends or some kind of server events but otherwise I don't touch them.

My main character has a lot of stuff finished so I frequently have down time on it... That's when I go do something on my alts. It all really depends how much time you have and how much you want your alts to be ready to do certain content

That said I love crafting and I've geared up omni crafter sets on all my characters but that's only for the crafter lovers, do not recommend for avg player (⌐■-■)