Evolution is not quite as easy as everyone assumes it is.

What do you mean by evolution being "easy"?

Evolution, at its core, strives for only one thing: propagation of the species.

Even if we agree to attribute intentions to evolution (rhetorically or otherwise), and to narrow the concept of evolution to adaptation through gene transmission/recombination/mutation/selection, you can just as well argue that evolution "strives for" stable ecosystems or, as you argue below, for a better habitat for our microscopic guests.

Taken to this level, it's a wonder that everyone isn't spending their time either at drunken orgies, or fighting each other to get into the drunken orgies.

Why so?

Somehow, "civilization", "culture", and "society" have guided human development away from that model - and I don't presume to know how it happened.

It doesn't surprise me that constructive competitiveness in a social setting is evolutionarily stable before shortsighted, greedy selfishness. Other less intelligent animals have developed social behaviours. The surprising thing would be that a species with more brainpower to spare wouldn't have figured that out eventually.

I don't mean to downplay culture as a factor. But I don't see a dichotomy between culture and "evolution"; I see fuzzy borders and much interplay between genetic and cultural evolution, in most of our history as a species.

The fact that culture evolves much faster than genes and is the overriding adaptation factor doesn't mean genetic influence is going away just as fast. Much on the contrary, the relative slowness of genetic evolution serves to explain why we carry such a legacy of behavioral tendencies so long after its original usefulness has faded.

Although most of us don't spend our lives in endless orgies and fights, it's easy to observe some ancestral substrate that makes many of us basically prone to promiscuity and domination (although I don't think those are the only evolutionarily stable heuristics, even in a selfish scope). The cultural rules that keep these instincts at line can't be wholly attributed to reason either; genetically driven social tendencies play an important part too.

Taken to the extreme, why have we even developed arms, legs, eyes, ears, and minds? Simple bacterial reproduction does just fine.

I don't get what you are trying to prove here. What is your explanation for the fact that we developed those things?

In fact, there is so much bacteria in our gut and sinuses that 90% of the DNA molecules in our bodies are foreign. One begins to wonder, then, whose purpose our human bodies really serve.

Again, that's a matter of interpretation. Applying the concepts of "purpose" and "serving" to these topics is like carrying our everyday physical intuitions when we are discussing relativity theory. I think it's often more a trap than a help.

With that caveat, I'd say our bodies "serve", at different levels, our own cells and mythocondria, alien microorganisms living within them, ourselves as individuals, other humans and animals for whom our net influence is positive, society as a whole, and the Earth's ecosystem.

Yes, even if we are screwing up Earth as we know it, we are "helping" it become Earth as our children will know it. I have no reason to believe Earth "prefers" either version. Whatever the outcome, it will have turned out the most evolutionarily stable one.