I guess thats when you build the client with good UX and charge for it. But it would have to be code under a license that is friendly to that kind of extension.
Good idea on charging for better interface. License might not be an issue, though. A cheat that many use is to run the GPL code in its own process, the proprietary one in its, and use a high-speed IPC method to marshal data between them. You can even do this as a pipeline with a proprietary UI on front, complex GPL in middle, and proprietary middleware as last step. Many models.