Randomized trials like these—that is, trials in which the intervention is assigned randomly—are the simplest and best way of assessing the impact of a program. They mimic the procedures used in trials of new drugs, which is one situation in which, for obvious reasons, a lot of care has gone into making sure that only the interventions that really work get approved, though of course not with complete success. In many ways social programs are very much like drugs: they have the potential to transform the life prospects of people. It seems appropriate that they should be held to the same high standards.
I strongly recommend this article. It truly captures the strong desire to intervene relative to the weak desire to measure results scientifically. I think that this combination explains widespread failures in foreign aid, education, and health care.
Thanks to Tyler Cowen for the pointer.