I think what matters in the end is Conway's law. Conway's is frequently misinterpreted as an observation when it's actually advice: Structure your applications/repos like you structure your teams. You're going to end up with that code structure anyway, so might as well save some time.
I expect lots of people on HN are working on systems with very tight coupling between client/GUI and server and no proper versioning between them, as is common in web applications. Hence the replies to the contrary: you're probably from quite different worlds :)
(Now, I personally think that maintaining sound versioning practices is a good idea even if you do have tightly coupled control of both the client and the server side. But that may just be me...)