It is a cool idea but I prefer a paper wallet. I don't want to depend on being able to access coinable servers to use my bitcoin. As was pointed out above it is a nice way for less savvy people to have a secure way to store their assets.
You don't need access to coinbase servers to transfer your coins. With their creative arrangement they can go completely out of buciness and you can still recover your coins.
My concern more has to do with theft by governments through taxation. Coins stored with coinbase in their vault will be subject to taxation and capital gains taxes when they appreciate. (I plan on paying this if I have any coins but others may not want to for many reasons.)
Multisig paper wallets seem like a better option for those that are savvy, or some of the new hardware coming out like -
http://mycelium.com/entropyand other hardware wallets can help secure people.
I am not ashamed to admit I am less tech savy. So, this seems like I good option for me. I moved half my bitcoin there. Though.... as pointed out, I would still like to see better tech come in the future. Things are are even safer, even easier, and even more private. So I think that when these things come, things like better hard wallets or paper wallets, then I will start to use them.