If you use Gmail, what happens if Google locks you out of your account permanently and without warning? (It happens.) What if they kill IMAP support and you rely on it? Or what if they simply start to suck otherwise? How easily can you move to a different email host? How much disruption will it cause in your workflow? Does your email address end in @gmail.com? What would have happened if we all switched to Wave? What happens if Facebook messages replace email for most people?
Proprietary monocultures are so harmful because they hinder or prevent you from moving away.
This is why it’s so important to keep as much of your data as possible in the most common, widespread, open-if-possible formats, in local files that you can move, copy, and back up yourself.3 And if you care about developing a long-lasting online audience or presence, you’re best served by owning your identity as much as possible.
Investing too heavily in someone else’s proprietary system for too long rarely ends gracefully, but when it bites us, we have nobody to blame but ourselves.