Why do so many healers not use specific abilities?

[EaMett]

I think it depends a lot on context and your co healer. If I use EOS HoTs and my co healer crushes them by topping everyone off, you can bet I'm not going to bother anymore, might even pull Selene out.
If my co healer is topping tanks after busters... I'll save my excog. etc.. etc.. Hell, sometimes you're just waiting for misama II to fall off so you can weave your indom (becaue you know there's no damage output on it's way) and the other healer tops everyone off.... Fine I'll just use my aetherstack to dps.

I find that pugs are often a bit of a tug of war between healers. We try to come to a silent agreement on who heals what/how whilest getting as much dps out as possible.

[HyoMinPark]

I think it depends a lot on context and your co healer. If I use EOS HoTs and my co healer crushes them by topping everyone off, you can bet I'm not going to bother anymore, might even pull Selene out.
If my co healer is topping tanks after busters... I'll save my excog. etc.. etc.. Hell, sometimes you're just waiting for misama II to fall off so you can weave your indom (becaue you know there's no damage output on it's way) and the other healer tops everyone off.... Fine I'll just use my aetherstack to dps.

This is why in any PUG I don't normally use AOE HoTs. Because they're always immediately invalidated.

I remember a v12n I had recently with a WHM cohealer (I was AST - I think I actually elected to go Noct; I usually use Di for A.Benefic after the split for my tank) who used the following rotation after Cosmo Memory: Medica II > Medica > Medica > Plenary Indulgence. I had actually popped CU to mitigate and give a HoT to tick up the party and ended up with it being 100% overheal on all but the tanks.

[Raikai]

Because they too lazy to think, since these abilities require proper planning and fight knowledge.

[KeriFawn]

If you play the game enough as any job you start realizing that in order to preform your best; you must use most of your skills kit. I use most skills but some I don't always need depending on the fight. I think it's more than "get good" it's more about understanding each of the skills and when they best apply.

[Gugumuni_Bobomuni]

Pretty late to the party but from a sch point of view about excog:

Using it is a dps loss in many cases, since in order to use it, i gota sacrifice an aetherflow which could be an energy drain. So, the question is, is excog NECESSARY or not. if you survive without excog since theres no 2 back to back big damage (like from o4s exdeath, with his thunder 3 tank buster followed by his pretty huge auto attack damage that were known to be letal), then, if excog isnt exactly necessary, normally, WHM got tetra and Bene for tank healing right after a tank buster, and AST got Essential Dignity, which would heal less if i actually used excog here. Also, as a sch, if there is no danger of tank death immediatly after a tank buster, Fey Union is a better use of resources so that the Aetherflow can be spent on ED and the tank will still recieve a crapton of healing from the tether, just, not immediatly.

Excog is very very niche in its raid utility, only good for back to back big tank damage, since if its aoe followed by tank damage, you would be using Indom if Hots wont be enough, and if the other healer isnt wasting resources, they should have a big tank heal available as mentioned earlier. The priority for healing in general is :
free resources healing (fairy stuff and cohealer)> limited resource healing (aetherflow) > casts
In most cases, in a well oiled healer duo, you shouldnt need excog unless things go wrong or unless a specific mechanic calls for immediate spot healing without enough time for hots or with damage fast enough that maybe the cohealer wont have the time to use an oGCD if he's in the middle of a spell.

I did use it in o7s on the missile bleed dps, along with fairy tether, might have been overkill here, but its good if you dont 100% trust your cohealer. In o6s the offtank did take significant enough damage that the sch had to use a bit of aetherflow to be safe unless well geared.

Excog has the issue of being a trade off for dps, using it means no ED, as i wrote before, so, unless its absulutely required, or unless its planned by the 2 healers planning their whole fight, don't expect excog in general. You might see excog being used as an instant 800 potency heal when things go bad instead of the actual post tank buster prep heal, but excog itself hasnt been a make or break thing in most cases. If you got time to be healed and not in immediate danger of death, Fey Union and Whispering Dawn should usually be good enough.