> It also gives you a shim into some filesystem/OS APIs that a regular sandboxed browser page wouldn't have.

If you're just talking to a localhost server, it's got all the local access it needs. The web UI doesn't have to do anything besides control the native service running. I'd much rather be able to ship a tiny binary and use the local browser than pack around a whole browser just to access otherwise restricted/sandboxed APIs. You can just drop a URL shortcut on the desktop (or wherever) during installation.