There isn't much incentive save for the fact that it is easier to configure. For some versions of DNS servers you need to consciously disable recursive lookups and then configure the list of servers that are accepted.
If a DNS server doesn't perform lookups for any outside server it isnt very useful.
If you think about a web host, with X number of servers that host websites, these are considered the Master DNS servers when the physical domains being hosted reside on those servers.
The public listed DNS servers on your domain WHOIS records are actually the Secondary DNS servers, and these perform the DNS lookups when someone accesses the hosted website on your Master server.
If the Secondary server accepts requests from anyone, even domains it isn't explicitly responsible for, then it is performing recursive lookups.
A more secure configuration is for the Secondary servers to only accept lookups for its own Master servers.
If a DNS server doesn't perform lookups for any outside server it isnt very useful.
If you think about a web host, with X number of servers that host websites, these are considered the Master DNS servers when the physical domains being hosted reside on those servers.
The public listed DNS servers on your domain WHOIS records are actually the Secondary DNS servers, and these perform the DNS lookups when someone accesses the hosted website on your Master server.
If the Secondary server accepts requests from anyone, even domains it isn't explicitly responsible for, then it is performing recursive lookups.
A more secure configuration is for the Secondary servers to only accept lookups for its own Master servers.