They are heuristically useful. The approach makes several critical assumptions. Mullen's early work, dealing with the other-total ratio, illustrated that the size of the groups matters. The basic premise is that the smaller the unit, the more salient it is perceptually. Salience influences cognitive processing.
Categorization relies on both prototypes and exemplars.
Cognitive processes that simply categorization likely introduce biases. Because prototyping reduces complexity, it generates more biases than exemplaring. Mullen and his associates have confirmed many aspects of this thesis in their studies of name calling, illusory correlations, and cognitive clustering. Dr. Hu's paper provides a convincing demonstration of the import of this model in an application to conformity and attitude change. The study is complex, but suggests that individuals in minorities were more self-focused and they did engage in more cognitive work.