nationalism as a concept is a good thing.
In fact its absolutely necessary for the survival of a country. You can widen that to any group structure.. a group will only survive against opposition from opposing groups if the members of the group are willing to place the groups interest above that of other groups.
But here's the problem: Nationalism only works with clear defined countries, boarders and national identities. It works exceptionally well to strengthen and bring together a country in times of crisis, especially when there is a clear adversary or opposition (out group).
It doesn't however work well with our modern society because we are pushing and pushing to remove the importance of, and distinctions between, nations. Mass immigration, re-defining what nationality means to the masses, more cooperation then ever before, international business pushing for all countries to function the same to generate bigger universal market places.. heck everything is coming together. Fashion sense, architecture, art, music, all are becoming globalised, rather then distinctly nationalised.
I live in a highly nationalistic country (China) - people here love their country, view it with pride.. they would die for their country. Trust me, if WW3 ever broke out, China will be fine. They have a populous who believe in their society and a government who will happily and willingly prioritise its citizens and way of life above all else. Compare that to the Uk. How many citizens would die for the UK? How many value its culture and heritage so strongly that they would fight willingly for the love of it? Given the amount of young/middle age people who can't even identify what their own culture is, I would be willing to bet the number is small.
That's the problem with nationalisation though - its useful im times of conflict and struggle.. but in peace time its almost irrelevant in the modern world. It pushes against the entire direction that our society and the world is moving in, and it draws a very very blurry line between pride and racism/discrimination. After all you cant prioritise one group, without also disadvantaging another proportionally.
In the end nationalism will die completely. Is that a good thing? well - yes if you like peace and a lack of conflict. But be aware that the price you will pay for it will be national identities. We are a long way into a natural evolution away from unique nations and towards unified and uniform states, and its not going to stop now. You will gain peace, harmony, and equality.. but you will loose diversity of culture, custom, tradition, and eventually (a long way down the line) language.