The multiverse smears me into a messy continuum of me and not-me. In this "least arbitrary" of preference schemes, it is not at all clear what is actually being valued.

If you are saying that the MWI is just a way of visualising probability, then we are back to:

"Making such a decision while not knowing how the probabilty will turn out works just the same classically, with no multiple copies involved. Evidently the decision has nothing to do with "valuing multiple copies" - and is simply the result of the observer's uncertainty."

Observers often place value on future possibilities that they might find themselves witnessing. But that is not about quantum theory, it is about observer uncertainty. You get precisely the same phenomenon in classical universes. To claim that that is valuing your future self in other worlds is thus a really bad way of looking at what is happening. What people are valuing is usually, in part, their own possible future existence. And they value that just the same whether they are in a universe with many worlds physics - or not. The values are nothing to do with whether the laws of physics dictate that copying takes place. If it turns out experimentally that wavefunctions collapse, that will have roughly zero impact on most people's moral systems. They never valued other Everett worlds in the first place - so their loss would mean practically nothing to them.

The "many worlds" do not significantly interfere with each other, once they are remote elements in the superposition. A short while after they have split they are gone for good. There is usually no reason to value things you will never see again. You have no way to influence them at that stage anyway. Actually caring about what happens in other worlds involves counterfactuals - and so is not something evolution can be expected to favour. That is an obvious reason for so few people actually doing it.

Maybe - from the existence of this debate - this is some curious corner of the internet where people really do care about what happens in other worlds - or at least think that they do. If so, IMO, you folk have probably been misled - and are in need of talking down. A moral system that depends on the details of the interpretation of quantum physics? Really? The idea has a high geek factor maybe - but it seems to be lacking in common sense.

Purporting to care about a bunch of things that never happened, that can't influence you and that you can't do anything about makes little sense as morality - but looks a lot like signalling: "see how very much I care?" / "look at all the things I care about". It seems to be an extreme and unbelievable signal, though - so: you are kidding - right?

FWIW, after this "explanation", I am none the wiser.