If we're going by GCDs than I can guarantee you that even in the average normal raid run with a random healer, my GCDs will be 100% or very close to it spent on my dot & filler spell.

While it's true that we use other buttons to heal, tell any dps they'll only get 2 skills, one a 30s duration dot, to press constantly while the rest is oGCDs they have to keep on cooldown but pressing them will lose value over time as people get better gear and learn the fights.
Or tanks that all their combos and fancy GCDs go into the trash bin, they're left with one filler and one dot and simply have to use mit and a few damage oGCDs but, again, their oGCDs would lose value over time because of better gear and better overall execution of the mechanics.
They'll be happy.

That is the state of healers: GCDs are almost exclusively spend on dps and any oGCDs you use will become less valuable quite quickly.

In one of my latest reclears, I spend 207 of 331 total actions (including things like potion and sprint) on Broil IV. 19 on Biolysis. And I still had some chonky overheal on some skills and could easily just not press them and nothing of value would've been lost. At this stage, I press them more out of boredom and not because anyone actually needs them.

The first unavoidable raidwide in p6 happens after 1min followed by more nothing happening for another 90s.
Carby also has huge gaps between incoming damage. Tree even has a full 2min gap at one point.