The reason why client side certificates have never gained traction because it is a pain to safely store/backup secrets.

In bitcoinland we are forced to solve the problem of safely storing secrets, and over the years we have come up with software and hardware solutions to make this safer and easier to manage for ordinary people. Solving this is paramount to the success of Bitcoin, and nobody has solved it before on a grand scale. 

I see no reason for forcing end users to use two different mechanisms for safely managing secrets.

I agree that using a bitcoin address for authentication purposes might be confusing and potentially linking your funds with your identity. So I am all for using something else than bitcoin addresses and bitcoin private keys.

With bip32 we have finally agreed on a mechanism for generating a hierarchy of bitcoin private keys from a master seed. A similar approach can be used for generating a parallel hierarchy for authentication purposes. 

- Jan