The argument that democratic demands are impossible to achieve in an era of capitalist imperialism was one made a lot by left-communists and council communists such as Herman Gorter during the crisis of the Second International and early Third International.
In a debate about the United States of Europe slogan, though Lenin himself thought it was "impossible or reactionary" (Trotsky disagreed), it did not follow for him that democratic demands in general were unachievable. To this categorical argument from Gorter and others, Lenin wrote that on “the basis of [this] reasoning it would be necessary to discard a whole series of points from our minimum programme as being impossible under imperialism. While it is true that genuine democracy can be realised only under socialism, we still do not discard these points.”
In 1915, he continued to argue that democratic demands, far from in any way weakening the struggle for socialism, in fact, serve to “draw new sections of the petty bourgeoisie and the semi-proletarian masses into the socialist struggle.” Nor did he deny that revolutionary-democratic slogans had lost their relevance, writing that “political revolutions are inevitable in the course of the socialist revolution, which should not be regarded as a single act, but as a period of turbulent political and economic upheavals, the most intense class struggle, civil war, revolutions, and counter-revolutions.” www.workersliberty.org/story/2013/04/12/what-do-socialists-say-about-united-states-europe)
I think that's the key point. Democratic issues are of crucial importance to the working-class, for the reason that more democratic forms of bourgeois democracy are often more conducive to building working-class organisations and, in the case of the national question, socialists addressing it with a class position independent of nationalists of all strips can undercut nationalism and drain the poison of national divisions within the working-class. In other words, unite workers' struggle requires a programme with democratic demands around which you can build the sort of movement that can go on to smash the state.