Actually, Wright's starting point is comment in _Class, Crisis and the State_ that as sociology grad student he was expected to frame his ideas such that non-marxists would take him seriously. I guess he's been pretty successful at that given he's director of research center and titled professor (Vilas Professor of Sociology) at U of Wisconsin.
Don't forget Serge Mallet's 'new working class': engineers/technicians/ technologists/educated labor forming principal labor force but not exercising direction and supervision. SDS's Carl Davidson was doing similar theorizing about this in late 1960s, including grad assistants and junior faculty in his grouping.
Of course, Marx refers to middle class a couple of times in _Theories of Surplus Value_, criticizing Ricardo for failing to focus on:
'...constantly growing number of the middle classes, those who stand between the workman on the one hand and the capitalist and landlord on the other. The middle classes maintain themselves to an ever increasing extent directly out of revenue, they are a burden weighing heavily on the working base and increase the social security and power of the upper ten thousand.' (TSV, vol. 2, p. 573)
and suggesting that Maltus hopes:
'...mass of the middle class should grow and that the proletariat (those who work) should constitute a declining proportion (even though it increases absolutely) of the total population. This, in fact, is the *course* taken by bourgeois society.' (TSV, vol. 3, p. 63)
M also refers to 'middle and intermediate strata [that] obliterate lines of demarction everywhere' in chapter on "Classes" in _Capital_, vol. 3 but, as we know manuscript was not finished.
And both _Class Struggles in France_ and _18th Brumaire_ are replete with various classes and class fractions.
Awareness of and comments by Marx about social stratification, notwithstanding, he emphasized class polarization. He might have eventually offered us stricter definition of class but it seems unlikely that he would have become theorist of 'non-polar classes'. Those, who in recent decades have prioritized looking at them have engaged in exercises in despair caused, variously, by 'failures' of working class movements, emergence of new groups on left, disenchantment with what used to be called 'actually existing' socialisms, and (in likes of Wright's case) pressures of 'academic marxism.' Michael Hoover