Well, if the only way to create the tokens was by paying the exact equivalent of a monthly subscription fee directly to SE, there wouldn't really be anything there to exploit (EDIT: Well, except that in typing this I realized that hey, item duplication is a thing that happens here sometimes!) At the end of the day somebody's still paying the subscription fee, they're just getting gil or items or mithra dances in exchange for paying it on somebody else's account.
It couldn't work on FFXI though, because we don't have a flat subscription fee here. Or will there be a token for one character, two characters, three characters, one character with one wardrobe, one character with two wardrobes, two characters with one wardrobe, and so on?
Maybe it would be viable by trading real money for Crysta, pushing the Crysta into FFXI, selling Crysta for gil, pulling Crysta back out of FFXI, then spending Crysta on a month's subscription fees, but... like, wow, that sounds like a lot of hoops to jump through, and I think Crysta technically has a real-world value, making handling it in-game really sketchy.
And that's ignoring the very real possibility that adding an item like that into the game could allow players to exploit bugs or sloppy code to generate large quantities in ways the developers didn't intend. What happens to FFXI if just one unscrupulous individual finds a way to create infinite Crysta, does so on one account and then launders it by selling off to a number of other accounts they control, then sells it for, say, 70% of face value? How much of the player base will buy that instead of paying Square-Enix, and how long could the game survive that drop in revenue?