TakeTheHighRoad:
I know of one engine OEM that very pointedly says that derate/flex has no effect on safety, but is strictly an economic issue. I think this is the general industry attitude, although some crews believe safety (failure rate) is enhanced with reduced thrust.
There is another point to be made: Flex thrust results in operating at a more constant thrust/weight ratio. This may be trivial or not. It means that the TO roll time to V1 & Vr is more constant, and the pace of cockpit activity isn't hurried one day and leisurely the next. Maybe the human factors crew can comment.