If people are too unique for the individuality of their construings to be adequately expressed through the methods adopted, then the previous successes of the model are hard to explain. For an individual to express to us a construct in language at all, they must share with us the concepts of that language. Discussing motivation in language at all would ordinarily be impossible without this sharing of understanding. So much for de-individuation. The postulation of intervening variables is a popular pastime amongst ethologists and psychologists.
People seem similar enough in their psychology to make it worth a try.
If the exercise is doomed from the start, then it will fail, and I may have learned something about the utility of the approach, but I will be left with the problem of explaining results such as are found throughout the literature on the Reasoned Action Model which seem to show considerable predictive utility of certain postulated intervening variables. David Regis, School of Education, University of Exeter, 1990.