Here's the problem: if the field was hidden we want to treat it differently. As of Webform 4, conditional fields are built-in to the core webform module. But looking at the submission object, I can't tell the difference between a field that is blank because it was intentionally left blank by the user vs a field that is blank because it was hidden by a conditional rule. In the latter case, we don't want to overwrite an existing value in the other database, because the user wasn't telling us "I want this field to be blank" they were in fact telling us nothing about the field because they didn't even see it.
For more information about the scenario see the original issue.