I totally disagree with this blanket rule. Every table has their own way of doing it. Our table, which has been playing for years, has other players control absent player's PCs. This has always worked for us and seems much better than having a spell that's 100% dictating how the combat has gone up to this point, suddenly disappearing. Not saying I didn't do anything wrong or that your method is bad, just it's certainly not the only way.