I'm not a friend of nationalism, but I believe that it's a trade-off: of you want to be open to immigration, of the kind that pulls in newcomers, inviting them to become a part of the place they move to, instead of remaining outsiders, you have to give them plenty of opportunity to identify with their new home. Of course these days, we see the American flag used a lot in ways completely opposite to this, but that does not change the great progressive value national symbols could provide.