I don't think I agree with all of what is being said on this thread. My suspicion is that the costs involved in flight testing and approval of flex data for contaminated runway performance is prohibitively high and manufacturers simply don't consider it worth doing. I doubt very much that using flex for contaminated runway operations for the aircraft I fly would decrease margins at all - the BR710 was supplied to Bombardier on the understanding that flex would be used wherever possible because it decreased the chance of an engine failure by a small margin. So if flex operations were certified for contaminated runway conditions you may decrease the performance margin by a small amount but you may also decrease the chance of a failure so doing the maths becomes quite hard. I don't think it is vital to get off the runway as soon as possible - nor does the manufacturer otherwise they would be requiring flap 16 departures to reduce the roll. I also think flex departures at light weights in the Global are safer because controlling an engine failure is easier and I think maintaining directional control on a contaminated runway would be easier flexed than normal perf. Obviously you need the runway length to do it and you need to comply with the climb performance but I can't see a technical reason why flex take offs would be less safe simply because you get airborne slightly later. After all, its only like taking off at a slightly higher weight and you wouldn't say that decreased the safety margins would you?
This may well not be true for other aircraft and I am only talking about the airframe and engine I know..