[gruseom]

I think there's another factor. Even if you have talent for both programming and UX, it's hard for the same person to do both those things well on the same project. To program well you have to know the implementation inside-out. But to do UX well you need to see the places where implementation complexity is leaking through and confusing users. For that, it's helpful not to know too much about how the sausage is made (not sausage in general, but this particular sausage). Once you are habituated to the internals, many details will feel natural and obvious to you that are anything but natural and obvious to users.

The best thing, of course, is to have the program's internal model and the user's mental model be the same. Eliminate contradictions rather than hiding them with complex mappings. ("Design is how it works".) But even then, there will always be much complexity that the user should not have to know about.