I tend to agree with Mike W on the definition of "commodity", ...
Yet, the abstract conceptual model includes, as moments of that theory,
the development of processes that bring about these facts. Two examples:
the process of centralization and concentration and the growth of the
relative surplus population. One could have developed a theory in which
capital comes into the theory (and the world) fully formed. Yet, we see
not only the presence of classes other than capitalists and workers (NB:
the inclusion of the book on "Landed Property" in the 6-book-plan) but a
*process* that leads to their dissolution, e.g. the process of
proletarianization (itself related to the processes of concentration and
centralization). Then, there is the question of what becomes of those who
are in the industrial reserve army? Unless one assumes state-assistance
(but the state-form was not developed in _Capital_) or charity, what
becomes of the army of the unemployed (other than employment for a wage
for the "lucky ones")?
I would express what I view as a possible difference in perspective on
this question as follows: you ask whether this is a moment necessary to
grasp the capital-form and its reproduction; I agree that is a legitimate
question but I also think that the theory must also be flexible enough so
that it is capable of grasping the development of what is contingently
significant for the reproduction of capital.
Marx didn't write Book IV (The State), but that didn't stop Gerrt and you
from examining the state-form in your book. So it's not a question (only)
of what Marx did or didn't write about. It's a question also of what is
needed both for the development of an abstract conceptual model _and_ to
explain the empirical facts of our time (if we are to assert that the
model has any relevance to our time).
While one could argue that worker co-operatives have been (and are likely
to continue to be) incidental to the reproduction of capitalist relations,
clearly petty-bourgeois farming and "informal sector" activities are
important for the reproduction of [some] specific capitalist economies.
How does one go about grasping these developments from your perspective?
In solidarity, Jerry