I hate it when I'm emailing someone with a deliberately long response and by the time I send, the thread has already gone on and I have to re-merge it.
Insecure in what manner?
To me it's no more insecure than a VCS - sure, anyone could edit anyone else's code, but version history was always there.
(but it's read-only, there's no rebasing, people still use old-school e-mail clients)