You can backup your data. You can have fallback tools and services. You cannot backup your identity. And if that identity is controlled by Google, you are tying your online existence for the duration to the foibles of a publically traded company with a shitty track record of customer support.
Tell me with a straight face that you know Google won't mess you around like this - not just now, but for the next decade. Hell, tell me you know your account won't be algorithmically disabled tomorrow. If your last name isn't Gundotra, I don't think you can. Why, then, are you taking that risk when the cost is so small?
.com domains retail for about $10/year, Google Apps Free Edition is here: http://www.google.com/apps/intl/en/group/index.html - you can keep the services you like while retaining control of your email address.
Disclaimer: I have an @gmail.com address that I'm suddenly realising is a sword over my head.