"I was already in the midst of assisting worlds other than my own. If that is, indeed, the goal, then I must surmise that the train has little awareness of what situations it pulls people from. Otherwise, it would have waited at minimum another day, until it was certain my side would have no more need of me."

True, it would have been worse to take him even earlier, but even with his planned role finished, he wouldn't be certain until it was over that he wouldn't need to step in again. He's not at all sure how he would fare in direct combat against the likes of Marluxia or Larxene, let alone any of the actual Xehanorts, but...at least he knew he could have tried, if they needed him.