Have you ever began to talk to someone and then realize that they are already in a conversation with someone on their Bluetooth headset? If not you will. Bluetooth is fantastic for hands free cell phone operation. They however need a call indication light. That way someone can see you are on a call before they start talking to you.
Several times I have come home and my wife was listening to someone on her Bluetooth and I interrupted because I could not tell. I happen to come in at the right moment when she was listening. A blinking blue light would have kept me from interrupting.
Here is what I propose. All photo cameras should go click when they take a picture, even the digital. All video cameras should have a red light on when recording. All Bluetooths should have blinking blue light when in a call but I am not going to hold my breath on this happening.