I don't have any person experience in this, but I know that guy who made the mod for Skyrim got a job at Bethesda. I think he actually wound up working somewhere else, I don't remember.
But as far as I can tell, it would be just like any other portfolio. It shows what you can do. So yes, I would say it would definitely be beneficial, but how beneficial would be definitely up to the company and the quality of the map. I would think that they would also want original code examples in the portfolio as well and a decent degree, but again, just speculation.