We categorize for different reasons in different contexts, and our categories vary accordingly. The context here is when, by knowing the category, we also no the individual entities that make up the category. If you tell me, "P is a bumblebee," I know a great deal about P without examining it. If you tell me "P is a worker" or "P is a grad student" or "P is an evangelical" or even "P is a woman" or "P is a Latino" you haven't really told me very much, concretely, about this particular P. This was the context of all my posts on this topic. You really know nothing about a particular grad student just by knowing she is a grad student - you don't even possess a statistical probability that she will exhibit this or that feature.
And of course working class is a crucial category in _talking about_ or theorizing social change, because in doing that we are not talking about individuals or claiming to know something about any individual or group of individuals. We are not talking about "Worker" as "Identity." Rather, we are taling about the abstract social relations which constitute capitalism as capitalism and not something else. And in periods of working-class militancy (or when, as the old terminology goes, the class becomes a class for itself) to say P is working class STILL doesn't tell us much about P as an individual, or whether she is a teacher, a welfare client, a systems analyst, etc - rather, it tells us she is potentially one of those in self-conscious motion and that she will (probably) understand an agitational slogan advanced in class terms.
An an observation on list practice. Too often on this list when posters refer to some category (grad students, leftists, academics, evangelicals, mullahs, what have you) the purpose is denigration.
Carrol