Derate is actually quite useful on contaminated runway and can allow to carry higher weight out of that runway than full rated thrust, as it lowers the Vmcg quite a bit, allowing a lower V1. I have seen double digit V1s on 737NGs that way.
However, derate is not available on the small busses, no idea about the big ones, and come to think of it the newly acquired canadian minibus. Which does not seem to be an economic disadvantage, seeing how they sell. Be that as it may, from what i was told in training, the reason for the assumed temperature is mainly a historical one, and has never been challenged and changed since.