Neither the man-page nor the output of --help mentions --allow-secret-key-import, although the error output when trying to import a secret key without that option does mention it.
At first I thought this was an error in our original backport of the fix to this bug, but it appears to be an intended omission. From the NEWS file in 1.0.5: * Secret keys are no longer imported unless you use the new option --allow-secret-key-import. This is a kludge and future versions will handle it in another way.