Actually do. Since he's using getmail, its making a copy to his local system. It just happens to be able to do that over IMAP. You are better off doing this than doing the same with POP because Gmail's POP deletes mails as you read them, which is non-standard. You could lose emails if there are network, disk, or other errors.
gmail's POP server doesn't delete mail. What it does do is disappear them from the POP listing (ignoring any client "leave messages on server" config), although even this is resettable through the web interface.
Ok you're right about it not deleting by default. But there is an option to delete mail after its been accessed with POP. And in any case Gmail hides the message after you've read it, but you could still have an error writing the message to disk and thus miss backing that message up.
