if an optional field is not inserted into the form, then its values will not be promoted to the sharepoint library (these fields will have blank values),
there is a way around for these
create more fields(having one to one correspondence with promoted fields of optional sectons) and instead of promoting fields from optional section, promote those fields, set the default value of these fields to the default value of their corresponding optional section fields.
set rules on optional sectin fields, so that any updation in the value will reflect in the new fields created.