There is a small security risk associated with running a public recursive name server. Those risks include the possibility of increased load or being used as being part of a DNS amplification attack
You will have extra load on your servers if they are public recursive name servers as people will end up using them. A non-recursive name server will just reply with the DNS equivalent 0f "I don't have the answer" if people try to use it instead of say 184.108.40.206 as their defined name resolver.
Additionally attackers can fake a from/client address when performing a DNS lookup as part of an amplification attack, where that fake address then gets the response. Attacks fake a huge amount requests and DDoS the faked address. Read more about them here: https://www.us-cert.gov/ncas/alerts/TA13-088A
You public authoritative name servers should respond only to requests for you associated zones.
Hope that helps.