I can't speak for others but personally I tend to go full throttle the first few times I clear some content and after a while just get lazy.

Most fights are way too easy once you are familiar with them and thus start to feel like a grind when you (have to) do them too often. How do you deal with a grind? By shutting your mind off and doing the bare minimum needed to succeed. Why carefully plan an earthly star if I just can go full braindead and throw easy aoe heals out there? I mean it's not like the mechanics don't give me enough time to do so.

So to me not using some abilities is more about not needing to and overdoing some content. Funny thing is, the worse my group is the more I stay on my toes and the better I play.