I don't need an audible or visual indication about the passenger seatbelt. None of the cars I've owned has an audible indication for the passenger seatbelt. Some have had a visual indication, which is fine. The passenger seatbelt latch is within inches of my right arm and I'm able to visually verify if it's latched or not.
If they wanted to make something useful, provide the driver with an indication of the rear seat passenger seat belts. Why? Because some kids forget to put them on or maybe don't latch them all the way. Turning around to check a kid that's behind the driver seat isn't the easiest thing to do.
If there were indications for the rear seat passengers, simply checking for seatbelt lights would be much easier (and safer, especially if the kid takes it off in the middle of a drive) but that makes too much sense. Instead they want to annoy you with the seat belt that you can see and touch.