Bring characters from other Final Fantasies into ARR is stupid, period.

But at least if it was Rosa or Terra or something it'd somewhat make sense. Those characters would fit in that world. Lightning comes from a technological modern world, and it's stupid that she's in the game.

I don't care if it's optional or not. This kills game immersion. Just like adding Yuna's outfit and Lightning's outfit into the game. I love Yuna's outfit, it's probably my favourite outfit in the whole FF series. I'd probably get it in the game too, as it wouldn't look too terrible in Eorzea. But bringing Lightning's outfit in? Then bringing Lightning herself in?

MMOs are about immersion to a degree, to some people more than others. If they had said from the get-go it was a Final Fantasy cross-over MMO, sure, cool. But that's not what it was supposed to be. I don't care how beloved a character is. It'd be just as stupid if Cloud, Squall, or Sephiroth showed up. Part of the reason a character can stay so beloved is the fact that they're contained to one game/compilation.