I'm not sure there is. A player who wants out can be toxic to the squad if he is forced to stay. A player who is put up for sale can try to prove his worth and give the extra mile in training to make it happen. Most of the time, a reason for a player to be cast out to reserve is disciplinary related. IE Tevez refusing to be subbed on, Suarez publically wanting to go to Barcelona, Joey Barton being Joey Barton, etc. The thought behind such decisions tends to be less related to a "he's not really in my plans so I'll make him train away from me". Its more of a "I don't want the rest of my squad around this sort of behaviour".
Now, unless Schweinstiger has outright taken the piss, I don't think he deserves to be cast away. You can say he's taken the piss by going to Germany etc under LVG, but I blame LVG more than I do the player. If the player is allowed to relax, he'll relax. If Bastian is ready to adapt to Mourinho and work harder, he should at least be given the opportunity to show that in training. My gripe is that if Bastian is willing to meet Mourinho half way and prove himself, why should Mourinho not do the same?
He's still a very good player on his day and can offer much needed experience to our midfield considering Carrick is never fit for a good run of games.
Its a bit of a cop out reason. Schweinstiger doesn't make his squad untrain-able. One extra player, big whoop. You have the likes of Januzaj, Wilson and Rojo in the first team training, and some of them seem to have one foot (reportedly) out the door.