Good point. Bluetooth allow one-many pairings - the computer can pair w/ Bluetooth speakers, mice, keyboard, all simultaneously. But it doesn't allow many-one pairings, such as two phones accessing the car stereo. I'm not sure whether a phone being able to connect to just one headset is true or not, and if it is, why.
I have 2 phones and 2 tablets. Sometimes, I may have 1 phone and 1 tablet in the car, making the tablet one of the music sources. Normally, if I am listening to the radio and a call comes, the radio is interrupted and I can answer over the car stereo. But there is no way I can listen to the tablet over the Bluetooth w/o preventing the phone from being transferred to the speaker should a call interrupt. Bluetooth needs to have a many:many capability w/o having to manually disconnect and connect particular connections