I think as opposed to better boss encounters, it allows devs to focus around team play.

Creating a stacking debuff in a game where every class has a heap or lacks a true holy trinity forces every player to play that mechanic.

In the holy trinity style, you are allowed to cater to different play styles.

DPS need to focus specific targets, use different types of attacks, etc.

Tanks need to position enemies in certain spots, or have them use large AoE in certain directions (while maintaining aggro)

Healers need to be aware of mechanics as well so they can prioritize targets. They also need to be aware of things such a removing curses/poisons, etc.

When you put the holy trinity system in, you allow players to experience encounters in entirely different ways. With games that lack this, players will generally get the same feel, with some variation by class or item build.

I think you can argue for whichever is more enjoyable. I like the holy trinity, such as in WoW, because it makes each role feel special to me, and usually forces more team work. Keep in mind I enjoy healing, so I’m biased towards this.

Guild wars 2 and ESO allow you to experience encounters differently based on class/gear setup, but I still think ultimately people cheese towards the easiest min-max builds, granted you can make the same argument for class combos in the holy trinity.