I was just reading an interesting article on the introduction of disc brakes on road and 'cross bikes, in the Belgian-Dutch magazine Grinta.
The most interesting argument was something along the lines that mountain bike tyres are able to absorb the power of disc brakes without skidding, but
that it's been found that 'cross and road tyres are too narrow to absorb the power of the brakes without skidding. Does this make any sense?
Seems like an interesting point: in short, disc brakes are likely to skid when used on road bikes.
A further point was that disc brakes have not caught on in 'cross yet, because of the weight problems, and also because true 'cross riders hardly use their brakes anyway
I read in the Cyclist this month that Colnago use a smaller 140mm rotor on their new bike for the same reason.