I'm surprised at your dismissal of my arguments Silu. As I understand it if you are playing this game "properly" you will be applying a lot of pressure on your opponents, and probably invading their lands. You cannot help but stumble upon more resources. Perhaps you also play maps with poor resource distributions??
And there should be no excuse to not trade for resources, unless you are truly in a world war. Do you never find yourself with duplicate resources to trade??
Well, basically what you said was "who cares about happiness, when there's other ways to get it". That's what I understood anyway, if you get red faces you can "just trade for resources or raise the culture slider". Sounds like you never get happy problems. Well, good for you; I'm not as lucky

Sometimes you have traded for all the available resources and still get red faces. Sometimes you start in isolation. Or rolled a fractal with a wide, but thin landmass. Or are forced to wage early-mid war against the SoZ. Or need to deny an early AP resolution. Or a thousand other situations where early extra happy at the cost of losing some chops can come in handy.
The biggest strike against that here is that it's enabled by Monarchy, a tech that already is an abundant source of happiness in these situations. Of course military police costs hammers and gpt (doubly so in Pacifism, a situation where you usually work less tiles anyway so you can leave more forests), whereas Preserves only cost worker turns, which may or may not be a more available resource. I haven't played enough PIG yet to get a feel of how they affect gameplay as a whole. Some scenarios where I think they might be of the most use are happy-resource-scarce 'Mids Repr starts, or health-deprived starts where you have to keep forests around for the health bonus.
An interesting thing to note is that Preserves are the only way to get a positive outcome of a jungle tile, without teching IW. This could be handy in isolation with some, but not overwhelming jungle, if shooting for the Oracle for example (so you go for Monarchy prereqs anyway).
BTW, by your definition I don't think I'm playing the game properly, I often find myself in a situation where I feel waging war is impractical for the whole game.
And surely you can spare a few gpt to make your whole empire happier (by trading for happiness resources).
As with everything, it's a tradeoff. If the happiness resource gets so many citizens to work that they outproduce its gpt cost, it's a good tradeoff. Otherwise it's just throwing cash in the bin. Same with using the culture slider.