They generaly overheat because touring cycles brake gradually down very long decents with very heavily loaded bikes, overheating causes pad material to seperate from the backing, discouloration and extreme warping of the disc, brake fluid boiling and in extreme cases caliper seals and disc hoses melting, and this does happen we have seen and done it.
Mountain bikers generally hammer down decents even pedaling in most cases and use brakes for short periods of time and to come to a halt or in emergencies, they fail on mountain bikes too and you fall off, but its all part of the fun!
We would anticipate that the size of disc and caliper needed to absorb all the heat of a fully laden touring bike would need the brakes of a motorbike and would be far too heavy to fit to a cycle, probably why we haven't included mounts on the tour etc, as we can't see a time when they will ever be a practical option.
