The more FF games release, the less chance there is for supporting characters to appear in the game. They obviously want to appease the entire catalog of mainline games and there are only so many characters you can dedicate time to.
Well, look on the bright side.
Assuming this game does well enough to warrant expansions/sequels (and let's face it, it'll happen)? There's always the likelihood of more supporting characters rounding out the cast for future games in this series. Not unlike how things worked for the Gundam Vs. line.
After all, even SE can only make new FF games but so fast. But if this becomes a franchise, they'll need to take other measures in order to keep things exciting and keep people playing.