Regarding the Dutchies: it was the RLD in those days, many moons ago. They didn't allow it on our B757/767's and I heard from HV guys that their B733's were all assumed temp of max thrust. I don't know for sure about their B738's, but I think they use assumed off 26K.
I'm not wanting to focus on that point; rather I'd like an answer as to why fixed-derates are a bonus. EU operators I've been associated with have 26K, 24K & 22K ratings. I hear some USA airlines have only 24K or 22K as max. They don't need 26K. If you need 26K on some occasions, fine, but why not just assume temp from 26K? Why have 3 sets of tables to achieve a thrust that matches the performance for the same weight. My performance dept claims to have it from Boeing as a good idea. No further explanation. I can see that lower thrust and lower VMCG might give a higher MTOW from contaminated rwys, but rarely unless very light weights. The reduced CLB could be calculated by the FMC as necessary from the amount of assumed temp. using fixed de-rates and then assumed temp just seems to complicate things and waste paper and printing ink. Save a tree.