If you keep your Bitcoins in Coinbase, You're aren't really in full control of your Bitcoins. You'd be better off buying a hardware wallet or create a paper wallet and keep it there. The problem with Coinbase could be..
Unavailability due to DDoS or some server issues in case of emergency and can also be hacked.
You can't sign a message to proof something.
You can't claim any new or all fork coin.