There should probably be a simpler means to add secondary NS records for folks who aren't using the Cluster Slave feature and who aren't using the local machine for DNS. It's a question that comes up a lot.
Part of the problem is that folks often don't realize how cool/easy the Slave auto-configuration is, so we should definitely mention that in the help for the new options...but it shouldn't be necessary to add bare records to the "BIND DNS records for new domains" field just to get NS records for servers that aren't under the control of Virtualmin.
I'm writing a new help page that provides a list of "Virtualmin is installed. Now what?" steps, including this information...but everybody expects it to be more obvious, so it probably should be.
I hate to add more options...but the BIND DNS domains form is pretty small, so it's probably not terrible.