What's consistent is the static healing requirement in any given duty. That is it, and you can always plan around that. You can't just dismiss the mistakes players can and will make. The dev team most certainly does not, so what makes you think you can? You CANNOT design content based on optimal play. You have to allow for some mistakes, or the content will not be able to be cleared by most groups. While you're not saying that directly, it is what is being implied in your statements.
RDMs and SMNs have very little healing ability. They can raise, yes. But if someone takes damage from an avoidable attack, and the unavoidable is about to finish them off, there is very little a RDM can do, and a SMN can't do anything in this situation. All they can really do is raise the player after they are KO'd. I absolutely should not have to point these things out. Anyone who plays this game and participates in PvE should be well aware of these factors that are present in everyday gameplay from the playerbase. At any given time, there is a player currently eating dirt that the healer could have saved.
As for your last statement, I really don't know what you're on about. You're admitting the truth in my statements, but at the same time don't really want to accept them and resorting to the rare mentality where players expect to be carried instead of learn. I understand that these players do exist, but I can also make a case for the impatience of other players who won't give these players the time they need to learn. You ever been in party that is clearly specified as a learning party, only to have one or more players leave after the first wipe? I for one can vouch that this happens far more than running into players who truly do expect to be carried.
Again, players love to only tell the part of the story that fits their arguments, when they are well aware that other factors exist.