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The overridden create_socket() method will have the same behaviour for the case when a socket map is *not* passed in to smtpd.__init__(). Users using the existing signature for the constructor will cause the sockmap instance attribute to be set to None, and this will get passed in to set_socket just as was happening before. So the only time the overridden create_socket will behave differently is if a non-None value is passed into smtpd.__init__(), and that's by design.
Of course there is a slightly increased maintenance burden, in that other functional changes to asyncore.dispatcher.create_socket will need to be duplicated in smtpd.create_socket. However, such changes would be fairly infrequent, methinks. A comment could be added to asyncore.dispatcher.create_socket if necessary, to remind maintainers about this.