We often fit simple models only because we cannot estimate a continuously changing set of parameters without vastly more observations than are currently available to us. Moreover, we recognize that the simple linear functions underlying most of our econometric structures may not hold outside the range in which adequate economic observations exist.
For example, it is difficult to have much confidence in the ability of models fit to the data of the moderate inflations of the postwar period to accurately predict what the behavior of the economy would be in an environment of aggregate price deflation.