The problem is that if you can remember a password, then a computer can guess it, and if you store something in the cloud then a computer can perform offline attacks all day long. And as time goes by, those offline attacks only get faster. This is one of the reasons that Mozilla's new Firefox accounts & sync system are insecure and unsuitable for storing passwords or other private data.
Sure, if it's an epic passphrase run through PBKDF2, bcrypt, scrypt or argon with a truly evil work factor (like, on the order of hours), then _maybe_ it's suitable for securing a backup of the keys to one's kingdom. Maybe.
Better, I think, is to encrypt the backup with a secure key, then encrypt the secure key with a memorable password, then use k-of-n secret sharing to give shares of the encrypted key to some number of trusted people. Up to k-1 of those people may disclose his share without endangering the encrypted secret, and if k do disclose it then it is still protected by the memorable password.
The problem is that in a world of cloud backups, a mistake tomorrow can endanger yesterday's backups.