I've always struggled to get any sort of phone connection when in the air, even at low level. But I understand others have.
It would be useful though none the less for making a quick call to someone that you're meeting at the far end to say that you've above to take off.
Also, with people who use their smart phones as a gps, it may be possible for the sound (warnings etc) to be routed direct to their headset via the bluetooth.
It might also be useful for listening to music which I know some people like to do. I'm pretty sure you can get bluetooth headphones now four your mp3 player, so it could be used in a similar way I suppose.
dp