There seems to be a misconception about what unpredictable damage entails. Damage can be unpredictable even in scripted fights. Take living liquid in TEA for example, the damage the party takes during jagd dolls can vary depending on how fast players feed their doll and how much hp the doll had. This can be, of course, optimized, but the fact that the damage taken is unpredictable remains.

Back when boss autos could crit, the damage a tank took was unpredictable. Think about the fausts in alex savage or the autos from a9s at min ilv... The fight was easy but the healer had to actually pay attention about tanks hp, rather ignoring autoattacks like we always do now.

if more fights had mechanics where damage varied in frequency and/or intensity, that would make healing more interesting even if the fights are still scripted.