The study of collective behavior and social movements has been a central sub-discipline of sociology since the 1970s. This is understandable for several reasons -- first, because collective behavior is inherently an important sociological process, and second, because the 1960s and 1970s witnessed particularly significant social movements in the US and other parts of the world. The US civil rights movement, the Vietnam anti-war movement, Czechoslovakia and France in 1968, and a variety of anti-colonial struggles in Asia, Latin America, and Africa made social movements particularly salient for sociologists in the 70s and 80s.
There are several different kinds of research questions that can be posed about social movements. One line of inquiry is descriptive and ethnographic. Researchers could immerse themselves in the concrete details of specific examples of social movements, discovering some of the specific characteristics and processes that were to be found in specific examples. Moreover, researcher could recognize the importance of failure and provide a similar level of description and narrative for failed social movements as well. This kind of descriptive research is very important in the study of any complex social phenomenon.
Second, given the interest that sociologists have in the explanation of social processes, it would be natural for sociologists to attempt to discover the causes of successful social mobilization. Comparative sociologists might approach this task by trying to discover some macro-social factors that would appear to distinguish successful from unsuccessful mobilizations. In other words, they might isolate a handful of examples of successful and unsuccessful social movements, and then use sociological theory and imagination to identify a set of macro-factors that might be thought to be conducive to (or inhibiting of) successful social mobilization. This strategy suggests use of Mill's methods to sort out necessary and/or sufficient conditions for the outcome. And it would issue in pronouncements like "successful social movements require X, Y, and Z as necessary conditions."
A third possible approach combines some features of both of these. This third approach acknowledges that each social event embodies a great deal of particularity and contingency -- thus requiring a substantial amount of descriptive research. But this third approach also postulates that there are causes of successful and unsuccessful mobilization and that these take the form of concrete social-causal mechanisms. So this approach directs the researcher to engage in concrete research with the goal of discovering some of the concrete social mechanisms that appear to have been critical. This research in turn has some promise in providing the basis for some limited generalizations in the study of social movements. If we find that there are some common challenges that efforts at social mobilization confront, this is a beginning of a more general treatment. And if we find that there are a handful of key mechanisms that recur in many cases, this further supports the development of more generalized statements about mobilization. (This is roughly the approach that McAdam, Tarrow and Tilly take in Dynamics of Contention.)
Now let's return to the role that resource mobilization theory plays in the study of social movements. This concept is said to be one of the primary theories of social movements. Its primary competitor on the 1980s was "political process theory." My question here is a simple one: in what sense do either of these concepts function as theories of social movements? If they are intended to serve as nouns in sentences like these -- "Social movements always occur in circumstances where there is more X in the social context" -- then I want to say that neither concept is likely to serve well and neither really functions as a theory of collective behavior. This usage is attempting to fulfill the second project above, namely, offering an account of the necessary and sufficient conditions of mobilization. However, given the contingency and heterogeneity of social events, it is unlikely that there are any such conditions. But the situation is much better if we take the view that both "resource mobilization" and "political process" theories serve to describe social mechanisms that are found in many different instances of social movements -- though often in different forms and levels of importance. On this approach, "resource mobilization" is a theory of a social process or circumstance that is a relevant causal mechanism in many different instances of social movements. But it does not function as a general theory of social movements; instead, it is a developed description of a social mechanism that can be recognized in a variety of contexts (not all of which involve social movements).
In other areas of science a theory of a domain is thought to be a compact set of hypotheses that explain all the phenomena of the domain. "Resource mobilization" and "political process" cannot function in this way. However, each of these concepts can function as a description of a limited but real social mechanism; and in this way they each can play a constructive role in explaining important instances of collective mobilization and social movements.