I specifically find Toxicon useful to open up movement windows, or if I've been forced away from a group of enemies due to AoEs, and still want to hit the whole group from a distance. I grant it's technically a DPS loss in a single target situation -- especially when you consider it costs two GCDs to get one Toxicon charge (and one of them isn't even affected by spell speed).
But I stand by my belief that it's got some fairly key uses, and I feel like burning an Addersting is still an improvement over just refreshing Eukrasian Dosis early if you need to move. Which, well, requires you to have an Addersting to burn in the first place.
Now, I will grant that I often cast the barrier on the tank between pulls rather than mid-pull, since it ain't a DPS loss if there's nothing yet to DPS. Or if I don't have any Addersting and mechanics force me into moving more than 5y from the boss (i.e., out of range for Dyskrasia) and Eukrasian Dosis has too much time left to be worth an early refresh, I'll often throw a barrier on the tank while I'm moving (because instant cast), so that it'll turn into an Addersting charge for when I inevitably have to move again later.
(If I'm within range, I grant that two Dyskrasia II -- for 340 damage even on a single target -- is probably a better use of two GCDs than Eukrasia + Eukrasian Diagnosis in order to get a 330 potency Toxicon II down the road.)
While I agree that "when" is pretty key (see above, about when I tend towards using the barrier and getting Addersting charges), I still think that some healers do have a need for GCD heals as part of their basic healing kit. WHM's lilies being the key example here. (Heck, the lilies are actually a DPS gain in trash pulls, provided you can get Misery fully charged and use it before the trash is dead.)
Now, I grant you that like 80% of the time I am burning a lily between trash pulls even when it's pure overheal, just so that I'm that much closer to Misery (and that's one cast of Holy I'm not spending on a Solace or Rapture instead). So, yes, "when" is important if you're trying to be optimal in your use of your kit... but it's not all of it.
I guess it's just that I feel like "if you're using GCD heals at all, git gud" is advice that is, at best, wildly oversimplified. I think we're largely otherwise on the same page.