A brain wallet and an encrypted paper wallet have the same problem. If you die and no one knows your password, the BTC are lost forever. Unencrypted wallets stored securely in your home are adequate for 99% of people. We're usually pretty good at securing things physically.
Just look at
Schumacher's accident, which is potentially a life changing event for him and his family. Having his key only in his brain may mean his
BTC are lost for ever. But having a plain copy in the safe is a different story.
I agree, plain keys in a physical security is going to be just as safe and much more reliable.
It depends what your purpose is... If some of you guys are putting life savings into Bitcoins and your family is ok with that, then sure you probably want them to access it when you die. However, there are others out there like miners who invested little or nothing and are just holding Bitcoins or for those casual Bitcoin investors who have < $2k in there, it might not matter.
For those who use Bitcoin because it's decentralized and you can keep the money yours forever even if the government seizes every asset you can possibly own, then perhaps this is why you DON'T want your private key ever disclosed. This is where the true power of a Brainwallet dies. The government can lock me up for 20 years, take every dime I own, but when I get out, as long as I remember that Brainwallet passphrase or can recover it somehow, the funds are mine.