I have seen a number of dismissive references to nationalism around (not just on DKos), and I think they miss the point. I think nationalism serves an important role.
First, let me clarify what I mean, and don't mean, by nationalism. Nationalism is simply an ideology focused on your nation. It is not the same as jingoism; i.e. being a nationalist in no way entails thinking other nations inferior, just as loving your family in no way means that you think your next-door neighbor is a lesser human being.
Now, why do I think nationalism is important? It teaches us to love humanity; it's one of the component in that.
We evolved a predisposition to love 'our own', whatever 'our own' means. This is what 'benefits' our genes. However, as Steven Pinker famously said, our genes can go jump in the lake. We recognize that we shouldn't simply protect our relatives and treat everyone else as means to our ends.
We are born with a predisposition to form attachment to our family. However, through loving our family, we learn to love our friends, those whom we make "our own". And then we expand this circle of humanization to our neighborhood, and our town, and our region, and our country -- and eventually, humanity.
It's like a series of concentric circles of humanization: as we develop, we broaden our understanding of what it means to be a human being, while not forgetting where we came from, precisely because our origins -- our families, towns, and nations -- provide us with grounding for further humanization of the world; each successive expansion of humanity, of intrinsically worthy personhood, builds upon the previous one.
Nationalism is simply an important stepping stone on this path. In embracing nationalism, we learn to recognize a whole nation as 'our own', and prepare to extend the same recognition to all of humanity, in the same way as through loving our family, we learn to love our friends and neighbors.
Nationalism is not destructive, it is constructive. In fact, I think the fall of USSR provides a great illustration of how important it can be. When USSR dissolved, outside Russia, pretty much the only republics which seems to be doing OK for themselves are the ones which had a history of nationalist movements: the Baltic republics, Ukraine, and Georgia. Belarus, ethically closely related to Ukraine and Russia, is stuck in a Groundhog Day version of USSR, in my opinion precisely because it never had a nationalist stage, it never developed a coherent view of Belarus as a single ethnic group, as a coherent 'us'; and so now it kinda just floats along, lacking any coherent impetus for progress, an amorphous society without anything to give it ideological and cultural shape.
Just like any other force, nationalism can be turned to destructive ends; but it is itself not destructive, and it's a very important force, one which is very difficult to replace. It provides a country with an important ideological core which permits further progress. It's not the only such core (USA used its Constitution as such a core instead, for example), but it's the most common one.
Thus, I think, we should not denigrate nationalism where it manifests, but embrace it and harness it to constructive ends.