The real power of URLs is for linking between sites, or linking to a site from another application. With HN as a website, it's easy for me to send someone a link to an HN comment thread over email or IM, and Hn doesn't have to specifically support that use case. If it were an application, that would depend on whether and how HN had implemented a "Share" feature.
Sharing URLs between machines is critical, but they don't have to be at the top of every window to be shareable. I agree that simple URL sharing should be a core feature of a browser - but a small 'share' button that copies the URL into my clipboard would suffice.
