Many people, including my friends, use passwords that sound exactly how they sound when responding to a question about what the password is. For example, if person X asks person Y what the password to the computer is, person Y responds "i don't have one". Now that is meant to be taken literally as the the quote is the password.
So should these types of passwords be used and what are their limitations? I suspect not for the same reasons why we don't use 1234 or abcd as passwords.