Why do DNS zone files require NS records?
Solution 1:
NS Records inside the zone are authoritative, the ones received from the registrar (the ones delegating the domain) are not, so if a server receives any difference from those records, the authoritatives win. Also, servers listed in the NS record will receive a NOTIFY in case of zone changes (so the slaves can check for updates and so on). One last thing, the RFCs 1034 and 1035 say that they must exist, so they must exist!