This is a difficult question to answer, because the culture of each company is different. If the company is oppressive and people are afraid to tell the truth, even when they leave (as they fear for their future careers), then exit interviews would not be all that helpful.
However, if a company has a fair reputation and the culture of the company encourages people to talk and be honest, then exit interviews can be extremely helpful. Therefore, what is most important is the culture of the company.
With this stated, there are a few other ways in which a company can determine why there is a high turnover rate. For instance, a company can have a anonymous survey of complaints that employees have. Something like this might give the executives of a company a better beat of what the people of the company are feeling or thinking.
In the end, exit interviews of fair companies serve a valuable purpose and can help in making the corporation better.