I am a huge fan of Final Fantasy, and have been since I first played it way back when in 1989, with Final Fantasy as my first Final Fantasy. Since then, I've played, to completion, every major Final Fantasy game that has since come out. I've also played Final Fantasy XIV from when it was released (I even have the collector's edition), and I've seen it change from a boring unplayable mess to what it is today. So, I know Final Fantasy.
If there's one thing that constant about Final Fantasy is change. Every Final Fantasy game changes, with each and every game changing something, doing something different not seen in previous Final Fantasy games, and this is what XIV is doing. Final Fantasy I started off being nothing than a D&D clone with a Japanese twist added into it, but it was a really well made game. In fact, most of the elements that we would identify being uniquely Final Fantasy actually started in Final Fantasy II. So, there's no set "story" or "lore" to the Final Fantasy universe, other than what the developers of a particular Final Fantasy decide what it is. The best part of this is that there's really no one telling the developers that they're doing something wrong from a lore-perspective.
With that said, I choose to have a position of faith when it comes to the inclusion of other Final Fantasy characters into XIV. My faith stems from two things.
The first is that I've seen how this game has changed from its 1.0 days to what it is now. This game is tons more fun to play, and I enjoy playing it when I get the chance (why can't Beta be up longer...). So, yeah, the changes made in this game have been nothing but good so far.
The second is that Yoshi is both sensitive and considerate of the concerns about putting in Final Fantasy characters into the game. He's already stated that characters are real, and that he won't do anything that would disturb how the different Final Fantasy stories came out. Meaning, we won't see Aeris rising from the grave anytime soon. Sephiroth on the other hand...
So, for those two reasons, I choose to have a position of faith with how things will turn out.