It's probably only for a few old songs, and why wouldn't Heath be okay? There's no way Taiji can actually take his job, and X has been very clear that this is a guest, not a rehiring. I love Taiji and both (perhaps Heath a little more... Palma, it would be lovely to help one Heath international fan from another how to get into their group *nudge nudge* ... I've been trying to figure it out for a very long time... *grumble* Then again, you don't has to.)
But, I see this as a way of making amends, and also helping out a friend that's in need. This may just be the boost that Taiji needs to get back into the scene. So, no one's trying to take anyone's job.
I can't even imagine not going or selling my ticket if Taiji played-- it's an experience with X that'll always stay with you. He'll probably play the songs that he created when in the band and do a solo before jumping off the stage and drinking a beer while watching the guys keep playing.
I don't see this as an old bassist re-joining the band for two shows, I see at as an old songwriter, who's compositions have not been played per his agreement with Yoshiki when he was fired, playing with the band for a few shows.
Let's face it, some of his songs are fan favourites. Heath's the bassist, there's no competition - but Taiji was a creative force in X, and was very much involved in an era of the band's history which has been forgotten for too long now.
How about hide? Since X reunited they've hardly touched his compositions. Scars was a staple in their live set and they only played it at the Hide Memorial Summit. It's about time someone other than Yoshiki had their songs worked into a set, as much as Yoshiki is the boss and X Japan is all about him - he's not taken the band to where it is now alone, and the fans know it and would like to hear other songs which he hasn't had a hand in, as believe it or not, they're actually pretty damned good.