This idea seems fine to me. I haven't cared how much exp my pokemon get, I just play to collect really. But for those who play for the leader boards and have others train their pokemon for them, this would hurt them badly. To help those people, you could make a function that people can use where they are able to loan a pokemon to another person, who can not take the loaned pokemon out of their party during the loan, and is automatically returned to its owner after so much time or the experience that the owner speficies is earned. The pokemon's owner would pay for the exp gained, not for the time.