Spir4: With a contaminated runway there is the situation where the friction available between the tyres and the runway surface is less than with a non-contaminated runway by an indeterminable amount. This will make directional control less certain due to the poorer lateral component available from the tyres - thus derating will help things (from an aircraft control point of view) but not minimise the ground run exposure - as you correctly identify.
Denti alludes to this. This was certainly available on the A330 and A340 and only really affected operations from short contaminated runways.