I'm sympathetic to the idea that working class -- class in general -- is fictive and useless, or at best secondary, but to say it's an empty category that is filled only during certain times of "militancy" is ridicilously idealist (and, not contradictorily, Leninst). If class is going to be used at all, it has to be used to describe what's happening, not what intellectuals and philosophers think should be happening.
For gods sake, man, either use it or don't.