There are valid points on both sides of this argument. It's true that the game shouldn't rely on the appearances of other FF titles' characters appearing, but I think a short-term promotional event to go along with the NA release of Final Fantasy X HD on March 18th (watch, patch 2.2 will be within a week of that date) would be just fine.

I do agree with the sentiment that XIII does need the promotional assistance, where-as X HD will sell itself.

Let us not forget though: Final Fantasy Tactics had Aerith in her classic role as 'the flower girl' and Cloud's appearance as a special recruitable character. Final Fantasy Tactics is definitely strong enough of its own accord, without reliance on their appearance, yet included them anyways.

How about a FATE fighting a Dark Aeon? One of the special perks of the International version that we NAs will FINALLY be getting.

That's just my two gil on the matter.