As an electrical engineer that has had to deal with RFI issues extensively in my career I am in full agreement that the devices should be switched off. The problem is that while a properly working device will have little or no emissions, a device that has been damaged can have significant emissions with no external indications.
Cell phones, laptops and other portable equipment are subject to dropping and other abuses. These abuses can damage components that are critical in suppressing spurious emissions. The devices may appear to operate perfectly normally or have minor issues such as poor range or reduced battery life. In the meantime they can radiate like crazy!
Better safe than sorry. Turn them off, I do.