Actually it is important.

I'm pretty sure you have died to 1% enrage, wouldn't you have loved if you only did more DPS during the fight so there was no enrage?

And why do we have DPS stances? Why do we get abilities like Holy Spirit, Fell cleave, Souleater?
You have those buttons as a tank, why not press them?

And even when doing ez content (expert roulette for example) is it required? YES, why shouldn't you try your best? Why shouldn't you use everything at your disposal?
Why spend 30 mins in an expert when you can do it in 20 mins if you only do your best and DPS more?
I do an expert every evening why would I let it take 30 mins instead of 20? Per week that's over an hour of more time waisted. If you count that on a full year it's over 61 hours extra, pretty much tbh.
I have a pretty busy life and have other stuff to do, so I'm a fan of speed-clearing especially when it's ez content.

With same logic DPS people should let themselves die and not use their defensive cooldown because hey why use a 'tank button' when tanks shouldn't use 'DPS buttons'?