I personally don't see an issue with crossovers. It allows people to have fun and enjoy some of their favorite characters in previous ff games with the entire community.
Does this "hurt" the immersion/lore of the game? Yes, maybe so. I don't see it being detrimental to the game though. I would be happy seeing people enjoy the game even if it means breaking out of lore every once in a while.

People on these forums spend too much time dictating what should and shouldn't be done. Just enjoy the game.