Exchanges imho are not meant to be used for long term storage of any significant amount of bitcoins.
Paper wallets are the best option for long term storage, preferably stored in a sealed envelope in a vault of some sort (yes even a bank vault).
With a paper wallet, your bitcoins are just as easy for your heirs to access as a stash of bank notes.
The question is not really about storage its about the ability to make sure who gets my BTC in the event of death. What is storage classed as 5 minutes or 5 years, it wont matter if I drop dead on a day trade expecting to get in a quick trade and out again. I dont have all of my holdings at an exchange but I want to trade when I want to so I leave some there. Look at Mtgox's order book they are holding over a million in cash alone.