I had FM radio on an old Creative Zen Micro MP3 player that I used years ago. Initially, I thought it was cool that it had FM radio on it, but afterwards, I tested it out and the reception was spotty and I rarely ever used it.
I think it's a terrible idea to "mandate" FM radio on anything. If consumers want it, they'll demand it from the companies making the products or someone new will come in and sell a ton of devices, basic supply and demand of the free market.
Making it mandatory would be a terrible idea.