There are a couple of problems with ssh keys:
No centralized way to revoke a key.
No way to enforce password policies on keys (is your private key encrypted, and if so is it encrypted with a good password?).
These are the problems kerberos solves. It introduces others, though, namely it is more complex to implement and requires a real user management infrastructure to deploy and manage.
In any case, although ssh authorized_keys allow for morris-worm type attacks, it is still better than .r services and telnet by miles (light-years)