It's a sensible system for deciding how to buy and sell oxen because it minimizes shipping costs. It's a less sensible way to, for example, judge the value of a person. Should I choose Alice of Bob just because Bob is at the beginning of a world-line and Alice is not?

This does kind of come back to arguing definitions. The common idea of identity is really useful. If a philosopher thinks otherwise, he's overthinking it. "Identity" refers to something. I just don't think it's anything beyond that. You in principle could base your ethics on it, but I see no reason to. It's not as if it's something anybody can experience. If you base your anthropics on it, you'll only end up confusing yourself.