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.