If the required rule is not used, your callbacks should assume that an empty value is valid. Otherwise, you would use the required rule, right?
On the other hand, I might have a callback which acts as a conditional required rule. I might check other values, either in the $_POST or the database, then determine that a field is required in this situation, and that the empty field is either required or not required. If the callbacks aren't run unless the required rule is in place, then I can't really do conditionally-required fields based on the other values submitted in a form.
One of my most common uses for this type of validation is when someone selects "Other" in a list of radio buttons or checkboxes, at which point the text input for "Other" becomes a required field.
