I agree with Eric that there is a difference between defending the *right* of workers (and all people) to move freely across borders, and embracing it as a positive condition.
I agree with him that the conditions of migration are generally forms of economic coercion.
But it might still be the case that the international disciplining of labour by the capitalist class might - for all the wrong reasons - have a positive effect. I mean of course that tendency that Marx identified for the ruling class to create its own opposition in drawing working people together, and homogenising their experience.
On an empirical note, though, the tendency is not solely for workers to migrate to the developed North, but in terms of the growth of the industrial working class, for new points of production to be created in the developing world, judging by the labour statistics. (see my article in the New Zealand magazine Revolution, with P Ray and J Woudhuysen).
-- James Heartfield