Now you made me want to make a rational argument for nationalism. Uhm... let's try this:
Imagine that some cultures do things that you consider horrible (e.g. genital mutilation, "honor" killing, killing people for blasphemy or sexual orientation, etc.). Your culture doesn't do that. However, for some reasons, the populations in the other cultures are growing, the population in your culture is not, people immigrate to your country and bring their horrible behavior with them. You are afraid that unless something is done against this, the repulsive behavior will become a norm in your country, too. Maybe not the majority norm, but still something that is more or less tolerated, because no mainstream politician would risk making too many voters angry. How to stop this?
Appealing to national feelings may be a realistic strategy. (The point is not to invent something that intellectuals would agree with, but something that has a realistic chance to get a popular support.) You can try to make people more proud of your local culture, and emphasise that not doing X is an important part of what makes this country great. Thus you get a strong political force against X.
Related: "Use Your Identity Carefully". -- My analogy is that nationalism is identity on the mass level (identity for NPCs?). It is a tool to preserve a group of memes, both good and bad. Instead of throwing away the tool, you can try to increase the proportion of the good memes in the mix.
That's certainly one, but there's an even easier argument to make - Burkean conservatism. Your nation's identity is built around stuff that's probably worked pretty well for it in past(or else it probably wouldn't be a nation today). As such, it's got a higher prior probability of working out in your local circumstances. It's not a large effect, of course, but it's a real one, particularly for successful nations.
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