John’s “single crack mode” (--single) is just about exploiting bad password practice — basing passwords off of the username, or details of the operating system. It can be used over “unshadowed” UNIX-style passwords (in which case it will exploit not only the username, but data from other GECOS fields), or on single hashes (in which case the hash must be written as USERNAME:HASH so that John has something to go off of).
Obviously, a wordlist is not required in single crack mode.