The thing to remember is that the information they are going to act on is the information that supports a change that HR might have been unable to sell to senior management in the past. It is not likely to affect policies that no one in management is interested in changing, and unless the whole staff quits and cites the same manager, it is almost never going to get a bad manager replaced – unless HR was already looking for a justification to replace him.
.. Any company/management that would actually change given the result of an exit interview probably would have been a place people felt comfortable discussing and attempting to fix their concerns prior to leaving.