You say no real benefits, but wouldn't you agree that open borders would would do a lot to minimise poverty? A higher GDP is often taken to mean a higher standard of living (as GDP is widely used as a measure of standard of living), so a higher GWP would seemingly also result in a higher global standard of living. I'm not sure you can so easily trivialise the economic benefits, even if your conclusion is that, as a whole, open borders are not a good idea.