Nationalism could be considered a very temporary force for good only if it represented a reduction of the sphere of state power rather than an expansion of it. In practice, that doesn't ever happen.
In practice, nationalism is worship of a nation-state, a Hobbesian artificial man. It would seek to prevent a body of people from seceding. Once a nation (aka a territory under which a government has exclusive monopoly on force) reaches some fictitious condition of sovereignty, imperialism is certain to follow due to the natural impulse of the state. Nationalism is militarism, expansion.
Why not abandon this horrid word? Let white nationalists have it. It's appropriate to their cause. It's a tainted word and always will be because of it's ultimate end, subjugation of the whole of humanity.