Raymond Williams has an excellent entry "Underprivileged" in _Keywords_, Rev. ed. (NY: Oxford UP, 1983):
<<<<< *Underprivileged* appears to be a very recent word, thought it is now common in social and political writing. It is especially interesting because of the primary meaning that had developed in _privilege_ (cf. PRIVATE), as a special advantage or right. It is true that the earliest meaning had been of a legal provision affecting an individual, and thence of a _private_ right. Someone might then be said to be *underprivileged* in lacking some such right or rights. Yet the modern social and political sense of _privileged_ had been so strong that this is almost certainly not how *underprivileged* actually developed. It can be seen, as it is sometimes used, as a euphemism for _poor_ or _oppressed_. But something more complex may also have happened, within a confused -- sometimes generous, sometimes illusory -- sense that _privilege_ is a normal condition. Compare the verbal curiosity of the assertion that 'we are all (or almost all) _middle_ class now'. *Underprivileged* is then a kind of special case, to indicate those falling below an assumed normal level of social existence. It is the assumption of what is normal that is then the problem, given the verbal continuity of _privilege_, which in its sense of very specific and positive social advantages *underprivileged* can have the effect of obscuring or cancelling.
The persistence of _under-_ formations may also have much to do with it. Compare _underdeveloped_, where the assumption of normal DEVELOPMENT (q.v.) is evidence of similar ideological certainties. _Underdog_, in that interesting phrase 'sympathy for the underdog' as an indication of humanitarian or even socialist sentiments, has a comparable but distinct formation, in its common use from 1C19. It catches almost exactly that combination of sympathy for the victims of a social order with the conviction or unnoticed assumption that such an order will or must continue to exist.
See CLASS, DEVELOPMENT, PRIVATE (p. 324) >>>>>
Williams' comments clarify why the dichotomy of "privileged" and "underprivileged" is not only useless but also obscurantist when used generously and generally. I make some _limited_ exceptions when used _very specifically_ to point to _specific oppressions_ that can't be simply reduced to class relations (e.g., white privilege, heterosexual privilege, etc.), *but* even then the term "privilege" doesn't help us much in understanding what we are up against. When used as general terms, "privileged" and "underprivileged" simply shore up the idea that it is only groups of "underprivileged" people who deviate from the norm of "middle-class" society. Again, Weberian notions lead to the narrowing of the meaning of the working class.
Yoshie