I had hoped they would be co-opted into the new Job quest set-up; a way to acquire assorted pieces of Job equipment or consumables. It would have tied in with the fact that each class seems to offer one job only, so the items you could purchase were a way of the guild that spawned your job being able to recognise and support you through your effort.

But, well, I guess the plans don't go in that direction. The above was based on the idea that those stories may tie into the guilds. Also, regarding a post somewhere above, I don't think a long term use for them is what Yoshi-P has in mind. His design indicates that they are redundant since the thing they purchase (at least for battle classes) is now obtained through the levelling process, and the questing process he had to introduce. I can't see that anything else can really be done with them without either skewing the current economy worse than it is for a while or putting them towards some other feature that doesn't actually exist yet (and in some cases, won't for months/years). No point having them hang around for spending on something that is that far off.

Can't really use them to purchase services that gil covers - repairs, for example, or airships/chocobo rentals, without the inevitable reduction in gil from guildleves that would go with it. I can see that they currently offer more scope for non-battle classes, since they allow for consumable purchase, similar to Final Fantasy XI. Whatever solution they hit on will have to have something to say about those current systems.

...I rambled a bit. Short of some class-specific gear, I can't think of a realistic use for them other than what Yoshi-P has already outlined.