Purchasing crafting loot from gems gained during SB and EW Fates is already a thing. So, there's one reason to continue, along with the other items one can purchase from the gem vendors.

The "nuts" allow for the purchase of tier-gear for alt jobs ... and a number of other items for glamour, et cetera, as well as XP for those alts. Some people prefer to get XP that way -- and yes, it's sad when all of your jobs at level 90 are capped and you already have better gear. But before that point, it's an easy way to go. And you can always purchase the tier-gear and sell it for the General Company seals.

As for the concept of a Bestiary ... sure. Let's do that.