I think Bluetooth is definately worth it. When buying the computer, it is usually like a $20-50 upgrade, and while I don't have any devices right now, I will get some in the future, and I like the option of getting them.
As far as the phone thing in the BMW goes, only the one that is connected to the bluetooth in the car would work. If both were connected, I would guess that they would both work. I don't know what would happen if you both were connected and your phones happened to go off at the same time.
Yes, bluetooth eliminates a lot of wires. See, if you have bluetooth built in, or even in a dongle, that in itself is the receiver for the devices. For most wireless devices, such as RF devices, you still need to plug into your computer a receiver, which adds a wire or two, even though it is wireless. Since the bluetooth module in your laptop would be the receiver, no external receiver is needed, and so therefore, no wires are needed.
The phone with bluetooth has several purposes. I guess it could be used as a modem, but most people I know use it to transfer files, sync their phonebooks, and the like. People also can transfer files between phones. This is useful for getting ringtones and images on your phone.