First, let’s face it, pure healer princesses are a rare breed. They make a lot of noise here but if you look at the ratio in game, they are pretty much inexistent (at least on Chaos). I see a specimen only now and then, something like once every 30 DFs and in PF their lifetime is around 3 wipes so doesn't matter. Thus, from my point of view, this an overrate problem. The real issue is that SE listen to the people making the most noise.

Now to address the problem, I don’t mind pure healer. What I dislike are dead weight. And a healer overhealing the hell out of the party or just standing there doing nothing is exactly that, a dead weight. And it’s not only healers, it can be no-rotation DPS, or no-aggro tank.

Currently if I face such a weight in a party, I do my best to prevent them to die out of boredom. For instance, as a tank, if the healer is a princess, I won’t use my cd nor avoid aoes.

OP, you’re right by saying that basic healing should be easy, and I think SE thinks the same. The problem is that the current implementation of this is: “heal requirement is very low”. It implies that the healer minimal job is almost non-existent, thus allowing pure healer to do nothing.

There is (at least) another solution to fulfil this requirement: “heal requirement is a full part job, except if you follow [other mechanism]”. [other mechanism] can be DPS, or cleaning buffs, or whatever, as long as you smash buttons. With such a solution, the healer has the choice. If he’s not confident doing something else, he can spam heal and it will work. On the other hand, if he’s confident, he can follow [other mechanism] to bring something else to the party.

Here are some examples on the top of my head to implement such a solution.
  • A damage buff on the mobs which is cleared if the healer attacks it. So, the healer either DPS or needs to heal more. The buff comes back every 2-3 gcd, it could stack too.
  • DPS ability with a very short (2 gcd) damage debuff. So, either you heal the none mitigated incoming damage or you DPS to decrease the incoming damage.
  • Increase overhealing aggro by a lot, so it prevents heal spamming and force people to be aware of the party status.
  • Increase the cost of all healing abilities (or disable auto MP regen) and add MP regen to DPS abilities. No DPS no MP no Heal.
  • You get aggro by not casting. So, if people do nothing they get aggro. (this is maybe not a good idea)

Idea which don’t force but encourage DPS:
  • Heal ability which potency increase the more you DPS.
  • Healer rotation, with 2 damages at the beginning and a big heal at the end.
  • Leech healing. Percentage of your DPS is converted to heal. (this could be the base for a whole new heal job)

Do you have more ideas to force heal to choose between easy heal spam or complex mechasim that brings something to the party?