When I lectured at the University of Canterbury, Associate Professor Seamus Hogan wanted to set an honours project asking a student to work out whether there really were any economic questions to which an economic impact assessment could provide a plausible answer.I would argue that there are two possible questions. The first question would be, What do economic consultants do get money to stay in business, while the second would be, How do we get a really big impressive sounding number to try to influence policy in a way we want.
I'm guessing the second answer is the motivation behind most studies and in many cases they work since the public is not critical enough of the results of such work. Which is why people should read Eric's article. to help develop a good BS detector.
If you were to ask the question, What economically meaningful question would an economic impact assessment provide a plausible answer to, then I think you have a problem. Its not clear there is an answer to it.