Flex would be bad judgment. Given a contaminated runway with unknowns, why give up performance?
DeRate is another thing altogether. As others have pointed out, it is like having different (lower rated) engines on the airplane, and everything that goes with it. And the key is that it includes a lower Vmc, which then leads to lower V speeds. The extra thrust from "bigger engines" is often not worth the effort/time/distance needed to get another 10 knots (+/-) of speed when plowing through slush.