why bother with the derate and assumed temp at all
(a) flex is limited by regulation to a percentage of rated. Ergo, derate plus flex provides for a deeper level of thrust reduction.
(b) derate may provide advantages by reducing Vmc and permit operations from shorter runways by virtue of a lower V1.
(c) especially with bigger (more expensive) engines, maximal use of derate may offer engine maintenance cost advantages to an operator.
I'm sure the experts in the engines arena will have additional reasons to add.