I don't know about your brokerage, but my fidelity cash management account sweeps money into a set of FDIC insured bank accounts, which is a hell of a lot more than can be said for your coins. And anything that is parked in a security is protected by SIPC from the brokerage going insolvent.

What will happen to your assets if coinbase's systems become inoperable or if customers try to withdraw more coins than coinbase has on hand? Ask Mtgox customers how they feel about where to park coins.