Right or wrong, one of the arguements used by the extreme free traders is that manufacturing jobs, do not offer or give upward mobility. Many even go so far as to argue that manufacturing is not the middle class, and is below the middle class. (i.e. working class, but not the poor). The arguement is that people "flow" upwards (in terms of wealth, not nesecarily income, though it very easily can be, inflation adjusted also) over the course of there lifetimes,these same people claim that the lone exception to this rule are people who work long term in manufacturing.
The big hitch and circular logic in all this however is that, eliminating the job, does not suddenly make somone middle class, instead you now having somone who had a job, but is not unemployed, assuming this person will automatically now "flow" is simplistic in its thinking.