Why would you leave your bitcoins at Coinbase? Nothing against them, but you should always secure your own bitcoins. The only exception might be day traders, who have to leave some coins/cash on the exchanges.
The problem I have with storing them in an offline wallet is that if I lose my private key I'm screwed. For me using an offline wallet is comparable to hiding your cash under your mattress, personally I feel safer by keeping my bitcoins in a web wallet. I'm just curious if Coinbase could get into any sort of trouble with the government if they decided to start stealing coins.
Encrypt your private key then store it in a bunch of places on usb-sticks, DVDs, etc. You van even email it to yourself if you use sufficient encryption and pw-strength.
Also, M of N paper wallets.