The option to disable the Manufacturing Mode interface is provided so as to avoid reprogramming/re-configuring the USB-Serial device.
Manufacturing Mode interface should not be disabled unless you will have no need to reprogram the device.
I thought that would be the answer. Unfortunately, our application doesn't like things when we have the manufacturing interface enabled (not sure why); so I had to disable it for our application to work. (I should probably talk to our developers about that; there really isn't a good reason for us to have to disable the manufacturing interface in some cases.)