This is further effectively covered in the Curator Guide where it is noted that if only a single subspecies is found in a location, and it is locally known by the same common name as the parent taxa, then you still should not add the name to the subspecies even assigned only for local use, because it still causes the same search issues.
The basic idea is that if someone is sufficiently aware they want to do a nominate ssp id, then it being listed under the scientific name is unlikely to be a barrier.