Again, the argument against the ability to use a mechanism because you wouldn't use it to protect the Space Shuttle's self-destruct mechanism, is unfounded. If a user loses his credentials, either in the sense of their being compromised or they're no longer existing, it should be easy to fix. With pass phrases, a customer service representative may easily change the user's pass phrase and tell the user the new one, during the customer's telephone call. The customer may immediately use that pass phrase to authenticate himself. With public key authentication, the problem can't be solved in one telephone call.
Similar issues arise for a newly signed up user.
Many service providers want to provide multiple services to a user, without requiring that user to possess multiple accounts or identities or credentials. Consider, for example, an Internet service provider that wishes to provide IP connectivity via PPP, and additional services. Ideally, the same identity and pass phrase should be useful for other services provided by that service provider, impossible if pass phrases are required for PPP but forbidden for other services. Such situations are no problem for pass phrases, but are more problemmatic for other technologies. Some day, every machine will have a smartcard reader, but that day isn't here yet.