I am bowing out of this, retired last year and really don't care anymore :-)
Congratulations, i really do envy you with another 20 something years to go...
As far as i understand a user selectable derate is legally the same as using pin programming to derate the engine permanently. So, while you cannot use assumed temperature (see the link to the FAA AC above, but i believe the EASA takes a similar stand) on a contaminated runway, you can use a different engine (rating).
Yes, it does not make sense if you just focus on the reduced thrust, but a derate has some other implications, for example using more thrust might violate the VMcx speeds, so pushing the trust levers up could be a bad idea, especially if taking advantage of the lower possible V1s. I have seen V1s below a 100kts on the -700, which might explain while boeing sticks to the 80kts call instead of the 100kts call airbus uses
