Andrew R Stewart 

I've got to support Stuart's understanding of the various aspects we'll talking about just now. I'll add that disk brake equipped bikes do break spokes on front wheels. We've replace more front wheel spokes then years ago and I can't remember the last time a rim brake ft wheel spoke broke that wasn't from an incident.

Do rims heat up during long downhills, sure and there are many stories about blowing off tires from increasing pressures, pads glazing and/or melting the contact surface (rubber is a poor heat conductor). But I can't ever remember a claim of spoke breakage from downhill braking with rim brakes. I have felt my own rims many times with my fingers, usually when I was curious about how hot they actually felt. This includes tandeming on the Blue Ridge and many tours with over 100lbs of bike and gear on my single (and with that kind of load you don't really go too fast but still have a huge momentum that loads the brakes). I have never had rims so hot that I couldn't touch them. Sometimes they were uncomfortable but not painful at all.

Now I have seen the results of disk rotor being way too hot. beautiful colors... Andy