The first reason Milo should not have liberated Rhyme and Reason is that he was not properly acquainted. Milo could have destroyed their world while trying to save them. Furthermore it was not his job to save the princesses. What if Rhyme and Reason were wicked people who were locked up for despicable reason?
The second reason Milo should not have liberated Rhyme and Reason is that he could have become unwillingly trapped in their world. He missed home terribly. What if he had died and never seen his family again? Thus no one would be there to guard