So you're saying that irrational beliefs aren't harmful?

I believe them to be harmful in general, yes. They can even be catastrophically harmful. Human-extinction level harmful.

On the other hand placebo effects exist, so untruth and irrationality isn't required to be harmful in every single specific case and every single specific incident.

I don't see how you went from what I actually said to what you thought I said. Please try to make as few inferential leaps as possible when evaluating the words of a stranger. On my part I tend to be precise in what I mean, and I certainly didn't mean what you thought I meant.

I thought the point of this site was to help people develop a system to reduce exactly such non-predicting "beliefs."

Among other things. It's not just about epistemic rationality but also about instrumental rationality.

Codifying the moral beliefs of atheists and believers could help to promote atheism as an alternative to religion

Personally I don't want atheism to be an alternative to religion. I want the absence of God to be treated as much a factual matter as the absence of fairies or unicorns or mermaids. There's no inherent "morality" in atheism, nor any inherent immorality either -- same way there's no inherent morality or immorality in lacking belief in mermaids. I'm sure that most evil people nowadays and most good people also, both lack such belief in mermaids.

That religions tend to confuse a factual issue (the existence or non-existence of various divine superbeings and their various characteristics) with moral issues, is one of the problems that I'd like to see solved, not contribute to its confusion.