You're acting as if I said I found it hard to dps in HW. I didn't. I never even remotely said that at all. So your explanation to me puzzled me as it was clearly based on something I did not even say.
I think most reasonable players do. If I see a sprout who is doing little to no dps, I may suggest that they can do more because mana in this game does allow for it, but I would never go straight into "omfg assist plz" like I have seen a few players do.
Design and clever use of game mechanics always had a funny relationship. A game is designed to do A, and players find B way of tackling it which proves more efficient and sometimes more fun. Just because a game isn't designed to be played in a certain way doesn't mean it shouldn't. In this case a healer weaving in dps is absolutely nothing close to a disaster. Nor is it difficult to do. Furthermore it makes the healer role have more depth and less idle time. Which is great in my books.
O2N has some of the lowest healing requirement I have ever seen in current max lvl content in any game when the group knows what they're doing. That being said I don't find dpsing in it fun because spamming Broil and refreshing dots is hardly compelling. It is still miles better than doing nothing though.
Sadly their approach didn't work well as the community answered with enormous backlash to the point of losing an enormous chunk of subs because the requirements to play well had suddenly drastically changed. I enjoyed it personally as the whack-a-mole healing in Wrath became dull to me but it was too much change at once. If they made every tier demand a bit more than the previous it would have been a very different story.
I'd say it's more the player's response to the same situation. Judging from what SE have said, it seems they intended healer dps to be fairly good so that questing wouldn't take so long to do, not to have something to do with excess mana and downtime in group content. Certainly in Wow for a very long time questing as a healer was very very slow and I think SE wanted to avoid that, but it created a side effect in instances which lead to a playstyle being considered optimal and normal...which lead to this thread.