The OS can't provide a key ring because the user dosen't use a password to login. Or if they do have an unlock code they are short and numeric, making brute forcing them trivial. So yes, in this case plain text is acceptable because there is no truly better alternative.

On one hand, both gesture unlock codes and longer alphanumeric codes, both of which android supports, would probably make good candidates to secure the key ring with. On the other hand, the email application, for example, still needs to receive email even when the screen is off, which makes anything entered on screen un-lock a bad candidate for key-ring solutions. Boot is better, but how often do you turn your phone off while not using it?