- Authentications where it's strongly in the user's interest not to disclose their authentication credentials, but doing so has little impact on the corresponding service provider. For example I'm probably going to be careful about my credentials for electronic banking (because I don't want you to get my money) and for Facebook (because I don't want you to start saying things to my friends that appear to come from me).
- Authentications where it's mainly in the service provider's interest that the user doesn't disclose their authentication credentials but it's of little importance to the user. For example authentication to gain access to institution-subscribed electronic journals, or credentials giving access to personal subscription services such as Spottify. In neither case is giving away my credentials to third parties likely to much immediate impact for me.
This is obviously a problem for service providers in case two, because it significantly undermines any confidence they can have in any authentications, and may undermine their business model if it's based on number of unique users. There's not much you can do technically to address this, other than using non-copyable, non-forgeable credentials (which are few and far between and typically expensive). It is of course traditional to address this with contracts and rules and regulations, but neither work well when the chance of being found out is low and the consequence small.
More interesting is what happens when you use the same credentials (SSO or single password, for example) for a range of services that sit in different places in this continuum. I suspect that there is a strong possibility, human nature being what it is, that people will make credential-sharing decisions based on the properties of an individual services and without really considering that they are actually sharing access to everything.
[I'd note in parsing a New Your Times article (Young, in Love and Sharing Everything, Including a Password) that suggests that young people will sometimes share passwords as a way of demonstrating devotion. I expect this is true too]