I hate the idea of companies giving exit interviews. The concept makes sense, but the reality is that they are 99% useless. I love the idea of stay interviews, which not enough managers are doing.
The stay interview is a much trickier one to do, because it requires a great deal of trust between the employee and boss. The idea of a stay interview is to have a frank discussion where you ask employees what would get them to stay and what would cause them to leave. The idea being that knowing these things lets you act. For example, if I know that the person values development opportunities, I can provide them.
Stay interviews can be derailed in 2 instances:
The relationship isn't built on trust, so the employee says what the boss wants to hear, not what is real.
The boss isn't skilled at listening and communicating after the meeting. The boss doesn't have to do everything mentioned in the meeting, but she had better not simply ignore it (or you will damage any relationship you may have with the employee).
Briefly - why are exit interviews so bad? First, smart employees are told to not criticize the company as they leave so they don't burn any bridges. As an employee, I'm going to make an excuse that doesn't burn bridges - usually say it was more money that was too good to refuse. As I've said, employees leave for money, it is almost never the primary reason. Second, most companies take the exit interview information and justify it. It is a common defense mechanism that prevents companies from moving forward.
My recommendation: Conduct stay interviews with each employee at least once every 18 months.