This I disagree with. Nations are real in the sense that they have a material impact on the world, but are fictions in the sense that they only exist when people believe in their existence. It's like money. Obviously money is real - I have some in my wallet right now - but if no one believed in the currency then it wouldn't exist. It would just be pieces of paper.
> Nations as constructed obviously provide value to the people residing there
I would quibble with the assertion that this is the _reason_ nations are constructed, but I do agree that they provide value.
Some people's lives are just more complicated. I'm a dual citizen myself, with two citizenships from birth. I was born in Canada from two American parents. I've had two passports all my life. I happen to live in the US but I've never really chosen one country over the other. I derive identify from being a citizen of North America. There are plenty of people with slightly more complicated than average stories, they should be able to exist.
But you asked, why remain a citizen? As a citizen, I can freely enter the US, even during a pandemic. When a relative dies, you want to go back and you shouldn’t have to worry about getting a tourist visa, you know? That’s not the only reason, but it’s a pretty big one.
But if it is meaningful, why should it not come with a cost? Nations as constructed obviously provide value to the people residing there, should this not come with corresponding responsibilities? Like paying taxes? And if there is value and meaning to being a citizen of a specific country why should a nation not place restrictions and requirements on this privilege? I could even see for example the incentive to ban dual citizenship as one. Why is it just assumed that keeping a bunch of citizenships, not paying taxes to that country, all while not living in that country is a thing that should exist?
It seems like these people want their cake and to eat it too. If the country wasn't good enough to live in, why would you expect to keep the citizenship if you leave? "Marriage" is a two-way street..