Martin Plissner says "the real problem is not that the exit polls were wrong. They were about as accurate as they usually are. The problem was that in the age of the Internet the exit polls were being seen by thousands of people who didn't know how to read them. Like any sophisticated weapon, they are dangerous in the hands of the untrained."
Update: Mystery Pollster offers a response: "If partial exit poll data is 'dangerous in the hands of the untrained,' and we choose to leave it lying around where the 'unsophisticated' will play with it, doesn't it make sense to at least publish a warning label?"