He gains plenty if he doesn't want to be a Devil and/or wants to pick where he can go. One more year and he can pull a Justin Schultz.
Justin Schultz had a way better college career than Jon Merrill. He'd have to make up the money he loses by not playing pro hockey next year (and subsequently when he comes off his ELC, as he'll come off a year later by staying the extra year). That's not easy to do.