Zooko's triangle[1] in action:

A BrowserID (like an email address) is memorable and secure, but not decentralized (it's centralized in DNS).

A cryptographic key is decentralized and secure, but not memorable.

Systems that are memorable and decentralized, but not secure, don't address these use cases at all.

(And then there's Namecoin[2], which is a fascinating development, but not likely to see broad adoption in the near future.)

[1]: http://en.wikipedia.org/wiki/Zookos_triangle [2]: https://en.bitcoin.it/wiki/Namecoin