How is the headset/speaker supposed to know which audio stream should be played. Assume you have two phones connected to your car via BT, listening to music from phone #1 and phone #2 gets called. Is the car receiver supposed to figure out which audio to mute and which to play, or just play both streams over each other and let the driver/user pause one?
Mostly that's a logic problem. Usually calls are prioritized over music in bluetooth, so if you were playing music on device 1 and a call came in on device 2, why wouldn't it make sense to pause playing on device 1 and play audio on device 2? That would be the "logical" choice for a relatively dumb playback device, but on a platform like a PC or something with a control plane for configuration choices it could be something that was configurable.
Mute playback on all devices, reduce volume to x% and continue playback, bridge audio to call and set playback to x%.
You could have choices for incoming calls similar to the call waiting prompts now on phones -- ignore incoming, accept and hold current call, or merge calls.
And the last obvious (to me anyway) option would be volume mixing choices for simultaneous audio streams to set levels for each audio device. I may want audio from the PC at 25% but my phone at 100%, for example.
Obviously simultaneous pairing presents some choices and not every device would or even needs to have options for every possible combination, but mostly I think there's default behaviors that would make sense most of the time for simple devices. But IMHO there's no reason not to have more configuration options if the device itself has some kind of control interface anyway.