Although the IV approach has been extremely influential in dealing with the endogeneity problems arised in many empirical studies nowadays (a notable example being that, to examine the effect of education on wage rate, Joshua Angrist of MIT and Alan Krueger of Princeton used America’s education laws to create an instrumental variable, say, quarter-of-birth, based on years of schooling), it is far from uncontroversial.
From the latest issue of the Economist, "Two recent papers—one by James Heckman of Chicago University and Sergio Urzua of Northwestern University, and another by Angus Deaton of Princeton—are sharply critical of this approach. The authors argue that the causal effects that instrumental strategies identify are uninteresting because such techniques often give answers to narrow questions. The results from the quarter-of-birth study, for example, do not say much about the returns from education for college graduates, whose choices were unlikely to have been affected by when they were legally eligible to drop out of school. According to Mr Deaton, using such instruments to estimate causal parameters is like choosing to let light 'fall where it may, and then proclaim[ing] that whatever it illuminates is what we were looking for all along.' "
However, "proponents of instrumental variables also argue that accurate answers to narrower questions are more useful than unreliable answers to wider questions." This sounds true, however, just as Prof. Deaton argues, and it also seems to me, that instrumental variables may encourage economists to avoid “thinking about how and why things work”. More deeply, "striking a balance between accuracy of result and importance of issue is tricky".