Alright, thanks for the opinions. I guess we can conclude that the two biggest reasons why BtT might not be in the game is because it wasn't that popular, and it's too "realistic".
The reason why I think it's a good idea is because, like I said before, it'll keep BBS' seriousness. Nomura said that BBS will have a very serious story, but that they'll try to use their talents to mix that well with Disney to please Disney fans. From that, I think Nomura has learned from his mistakes in KH2 that he shouldn't keep using random worlds that have virtually nothing to do with the story. Even PotC was a little disappointing. So, I think what Nomura is trying to say is that he'll try to keep the Disney worlds in line with the actual plot of BBS, and that he'll try to make it so as to satisfy Disney fans at the same time. If Terabithia was indeed in BBS, I've stated the reasons of how it would fit in the game, and I think those reasons really would please the fans; Nomura would probably plan out similar stuff if it was in the game. Finally, Disney itself would have its popularity increased as an outcome, and not just from BtT, but the other Disney worlds as well, giving Disney another good reason to give SE permission. A bit of a jump for the PSP sure, but Nomura says he's going to be taking advantage of many things, and I trust Nomura so...