There are various borderline cases where doing AOEs seem to not help. In such cases I hesitate to say "if you don't use AOEs you should go blast your head off."

1) When I see tanks being overwhelmed with pulls they cannot really handle / heals not being able to heal quick enough -- in such cases focusing attacks on a few to reduce the number of attacking heads seems to help better.
2) When I am playing as RDM, after my mana gauge is over 50 I do my 3-melee combos so I can unleash powerful AOEs. It would be a problem if people say those melee combos should use the AOE version which has a higher mana cost.
3) There are cases where tanks fail to capture all of the critters as they pull; in such cases I can die fairly quickly when I use AOEs. After run-ins with a number of crappy tanks I can imagine some people will hesitate.

Overall I would agree that AOEs should be used in optimal conditions.
However, as we can end up with various types of people in the roulettes I tend to not assume and at least for the first 1 or 2 enemy groups try to understand how the other party members are functioning.
If I can mindlessly blast AOEs that would be easiest, but there are cases where it may not be the optimal choice.