To me the purpose of models is to help humans manage code. Our brains don't hold much at once, so the more we can forget about in a given circumstance, the easier it is. So I think btilly is correct: the reason I want modules, ignoring things I don't care about, is the same reason I want them to deal reasonably with circular references.
In the example that I gave, the design described will handle complex edge cases such as a part time employee working for multiple companies. And will do so without programmers having to think through the whole system at all points.
Independence of modules has no importance in a codebase that ships as a whole. But modularity does.
