I haven't read the discussion ^^; (lazy bitch i am) but I think that Taiji was part of X, as it was then... when X became X Japan it went into a completely new direction and that is not the music Taiji seems to like or like he wants to do.
Actually I had no idea that Taiji even COULD join X Japan now, because it seems impossible to me!! Not a single thought about that...
I think Taiji had of all members a very hard, especially unthankful time... and I don't think he ever wants to be part of that band again. He sure had a great time, but surely a time of trouble too, especially with Yoshiki.
X was a band of 4!!! band leaders! And the worst fight was between Yoshiki and Taiji (especially cause of their opposite musical directions) and i think there's surely still a lot of bad blood.
He's a musician of his own now. Independent from X and everything that surrounds it and I'm happy he never gave up.
Since X doesn't exist anymore, there's no reason why Taiji should join them... even less since hide, who was the second person in the band going more into the hard, ballad-less direction is not there anymore...
