Coinbase, while a lot people use it as a wallet service, is mainly an exchange service, so you're not really convincing here. On top of that, Coinbase doesn't allow you to gain access to your private keys, which in a situation where the service for whatever reason stops working, you completely depend on the service to come back in order to withdraw your coins. In times where people directly suspect that a service got hacked after going down due to unexpected maintenance for example, it would be great if you had your private keys to keep yourself calm in such situations. In case you don't trust it at all, you can just move your funds. Remember, if you don't have access to your private keys, you don't actually own any coins at all....