Oh, I hope Xigbar's important because he is amazing! I love/hate fighting him, because he's extremely hard with all his laser bullets and changing the shape of the arena...but you can't help but think, "
Cooooool!" His whole personality is hilarious: while the rest of the Organization is trying their best to look tough and powerful, he gives off an attitude that says, "I'll look bad when I FEEL like it!"
Anyway, he does seem important, not only because of what he said about the other Keybladers in KH2, but because of the extra scene in KH2Final Mix. If you ask me, he seems
very interested in what Xemnas is up to, which Zexion notes: "You love to eavesdrop!" He even tries to form an alliance with Zexion. The only other times this really happens is when Marluxia and Larxene partner up to overthrow the Org, Vexen, Lexaeus, and Zexion partner up to stop them, and when Axel and Roxas become friends. Other than that, it seems to me that the members stick to their own agendas. Now, Xigbar strikes me as a pretty observant person, and I think he would know that it's better to keep your thoughts to yourself, since you never really know who you can trust. So I don't think he would confide in Zexion, one of the most loyal members of the Organization, unless he thought what was happening with Xemnas was really important. Maybe he suspected that Aqua's armor was in there, and that would be significant to him because of events in BBS. With all the good specualtion going on here, and the evidence that's been unearthed, I'm almost positive he'll be in BBS. As for 358/2 Days, we definitely know he's in that game, and he had some type of relationship with Roxas. Most likely he annoyed Roxas with cryptic messages, but there's a very interesting piece of dialog between them in that story "Another Report":
“Well, never mind. Perhaps we should ask the opinion of our own Keyblade Bearer?” said Xigbar as he gazed at Roxas.
“Well, I……”
He didn’t have an opinion. Keyblade…… the key to the worlds.
All he knew was that he had the same power as him, no more, no less. He was with the Organization because he had nowhere else to go and wasn’t even certain he wanted to retrieve his lost heart.
“Sora.”
Roxas looked up at Xigbar’s unprovoked call.
Sora?
He felt he’d heard that name before.
“What on earth’s that?” asked Demyx.
“Haha! I see, you don’t know anything,” chuckled Xigbar.
*cracks up* Oh Demyx...
So here's my theory: Xigbar will probably be the one to keep hinting around to Roxas who his Other is, and that will be a big factor in what eventually makes Roxas want to leave. I don't think Xigbar intended for any betrayals to happen, but nevertheless, he must've had some motive behind it.