I would imagine that Bluetooth tethering over WiFi or USB would be a matter of convenience, or if a device doesn't have the appropriate ports for USB. Another thought, MiWi OnDemand (jailbreak only), when being used between multiple iOS devices, will connect to a paired iPhone for internet connectivity over Bluetooth only when needed. I suppose that Bluetooth is more on-demand whereas using your phone as a constant WiFi access point would drain the battery considerably faster.
Ok, so the problem here was that somehow my iPhone was connected to iCloud... But it wasn't at the same time... This is really weird, and Apple should at least notify on this somehow, because my iPhone was connected to iCloud and synced. Wow.
Worked the same for me, though also required e) after a) to d):
a) going to the apple discussions (as mentioned) to 'register' my device(s) with my apple id. (weird)
b) turning iMessages off and on (on both devices)
c) turning my icloud off and on (on both devices) (scary dialog boxes, push on)
d) turning by bluetooth off and on (on both devices)
and finally, after none of that worked,
e) turning both devices off and on.
Presto, it finally worked. The feature only seems to work if the email or message or calendar (whatever you want to hand-off) is actually open on the screen on one of the devices. Unlock other device and the email or messages or whatever icon appears bottom left. Swipe it up and roll.
I have had a few of these issues with icloud and apple id's. Seems like you really have to reset all icloud and messages services to get hand-off working properly.
Best Answer
Certainly on the Airport range, MacBooks, and iOS devices there's no QOS, Apple doesn't support it. Not sure about the Mac Pro or Xserve.
EDIT: In response to requests for sources, it's very hard to find an official source for something that isn't implemented, but a simple google search for "does apple support qos" will result in many links to posts about someone asking the question about various Apple products, several on Apple's own support communities. Everything points to no QOS because it simply doesn't exist on Apple products.