First, as security breaches go, that's pretty minor. Someone could also shit on my front doorstep too, but somehow it doesn't happen that often. Second, there are better ways to link devices than pairing; for instance, hit a button on the receiver to see the closest device by signal strength, while also remembering all previous devices. Or, a UI for booting out the device ID of an accidental linkage or ne'er-do-well.
Pairing makes sense for sensitive data usage, such as tethering. But it's a broken pattern for consumer purposes like multimedia and gaming.
It's not that big of a security breach but it'd absolutely be annoying is anyone could do it on the train. People don't steal your mail but as soon a firesheep came out a dozen jokers a block were logging in to people's facebook.
Make it so it doesn't interrupt your headphones and then all of a sudden it's trivial for anyone to listen to at least half of your telephone calls.
Most BT headsets already pose a serious everyday security risk to people who might be targets of surveillance by as minor players as private detectives or journalists due to both protocol and chipset weaknesses. BT security has been improving since then, but the last thing it needs to do is go backwards because you can't be bothered to pair a device with your car once(!)
Pairing makes sense for sensitive data usage, such as tethering. But it's a broken pattern for consumer purposes like multimedia and gaming.