I want to start off with saying that I don't bother healers or tell them what to do in the game (unless they are missing a mechanic). When I see a healer who's just standing around I'll either drop or just keep going because I imagine if they're standing around at 70 they simply don't care to utilize half of their skill kit. That's their choice, I accept it. (Heck, it's half the reason I play a healer so often is because then I know the healer will be as active as possible. The other half is because I enjoy it so much) That said it's clearly a better option to use the spare time to increase outgoing damage than to do literally nothing but wait. Unless you're fearing that there's going to be the tankbuster of all tankbusters approaching then you're just slacking it. (Also I'd like to add that I'm talking specifically about standing around and doing nothing waiting for a heal to matter)
I don't think we should be basing things off what you 'must' do but what you 'can' do. I try to improve my damage as a DPS and using the right mitigation skills at the right time for a tank as well, but there are plenty of people who won't and things will still work out just fine.
As for the games brought up, I understand there's no pure healers in a lot of them. A lot even have builds (In RO I played a semi battle cleric/support so I recall doing damage plenty). There's also MP limitations or simply not having the ability to damage certain creatures. But if you DO have the methods and ample opportunity, why wouldn't you take it? I imagine the answer is 'because I don't want to', which is definitely an answer although a pretty selfish one.
Hell I play a PLD. They're a tank. Do you know how many times I've healed someone else because both healers were dead or simply not paying attention? If I had the mindset that I should be doing my primary role then I should be using that MP to spam flash s'more. Then that person would be dead because while I had the means and the opportunity to assist and greatly help with the completion of the duty, I decided 'nah'.