For instance, A writes to B asking a question. B passes on the email to C, and adds, "As is a freaking idiot". B and C have a quiet laugh at A's expense.
10 years later, A does a search on the email dump and finds that their erstwhile colleagues B and C have been stabbing him/her in the back all along.
I expect this dump to ignite more scientific feuds, hatreds and flame wars than any other hacking event in the history of man. Suddenly, anyone can find out what these people really thought of them at the click of a mouse.
The "scientists" at the CRU shouldn't be too worried about the response from the sceptics - their real worry is going to be the mob of axe-wielding, pitchfork-waving fellow believers who are incensed at what they've read, and who are going to be arriving at the CRU shortly with lengths of rope and burning torches.