If every work that discusses reality under the rubric of rational self-interested actors carried this disclaimer, we all would be saved a lot of grief:
Since human nature is profoundly complex and individuals rarely act out of unmixed motives, the assumption of rational self-interest that I have been using to develop this theory is obviously much too simple to do justice to reality. But the caricature assumption that I have been using has not only simplified a forbiddingly complex reality but also introduced an element of impartiality: the same motivation was assumed in all regimes. The results are probably also robust enough to hold under richer and more realistic behavioral assumptions. 
Of course, the real debate comes into play when you consider whether this is a good idea to describe reality. I’m not throwing my featherweight either way at this stage, but this is a nice statement of the reason rational choice assumptions are used.
 Olson, Mancur. “Dictatorship, Democracy, and Development.” The American Political Science Review 87, no. 3 (1993): 567-576.