Agreed. Another compelling reason is data deletion -- accidental or malicious. While catastrophic loss due to hardware seems to have declined over the past few years, malicious deletion has gone up markedly.
I've pimped them out before, but this is why I set up backupify (handle more than just gmail, since your Google password is for all your Google accounts) and periodically export my data. I just wish they did more, like backup my Dropbox and Atlassian accounts.
If there isn't much that you are worried about losing, you can just save a local copy from your browser. I only do that with a few per month, but it works fine with no extra fiddling.
IMAP is just a protocol. You're talking about syncing over IMAP. You can just fetch over IMAP too.
Also, POP3 is not folder-aware. You can only access your inbox over POP3. If you want to backup anything (e.g. drafts, sent mail, etc), then you need more than POP3.
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.
I just run getmail which retrieves mail via Gmail's IMAP interface every now and then. If Gmail went away tomorrow, I wouldn't be _that_ sad.