Well, I think passing and rebounding training is different, for two main reasons:
1) Every player on a team needs to be able to pass. Not every player needs to be able to rebound. You can still have an effective basketball team if your PG doesn't rebound. However, you can't have your Center passing the ball out of bounds all the time. I'm not saying the PF/C need to be the best passers on the team, but surely the C is going to touch the ball during an offensive possession and need to be able to pass to a teammate.
2) IMO, you can train guards out of position easier than big men. If you play your PG at PF or C on offense, you can play him back at PG on defense. On offense, he doesn't need to get the ball in the post or be a scorer offensively. No big deal, he's just not as involved in the offensive scoring. He won't take inside shots. However, if you play your Center at PG, he HAS to handle the ball and pass the ball. He HAS to be involved on offense because the offense starts with the PG position.
So I don't think it's the same comparing rebounding and passing.
All that said, I like the fact that training in BB is challenging. And I train my players out of position all the time as well. But I think the training aspect of the game would be improved if passing training was at least allowed for Wing players. Playing your C at SF is a sacrifice, but can be accomplished.