What happens when Google suspends your account and you loose all your email, docs, etc? Your online identity (you no longer have your main email address) gone?

Own your MX record and use forwarding and/or Google Apps for Your Domain?

That's what I do, and it gives you a portable email address, but doesn't address the other concerns. Ultimately, email is a communication medium, not a filesystem. Using IMAP folders for long-term storage is tempting, but it's off-label.

Aren't you a little bit uncomfortable giving Google all that data? Do you keep a back-up??

Not even a little bit.

Total number of times Google has lost my data: 0

Total number of times I have accidentally lost my own data: Way more than 0

Total number of times Google has in any way caused me harm by having "access" to my e-mails: 0

Total number of times I've been glad Google used intelligence gathered from my e-mails: Many (pre-populating Google+ circles, training a great spam filter, Priority Inbox, great search, decently appropriate and well-targeted ads)

Mail.app is keeping a local backup (using imap)