Software defined networks are big in the data centre: there are many startups because that's where the money is. But SDN is also very useful in the wide area.
Take the vexed issue of SIP handsets at customer sites. What we really want to do is admission control of those handsets' traffic into the Voice QoS class. Then hacked softphones can't run a denial of service into the Voice QoS class.
There's a whole IETF architecture to do this: integrated services. But no one uses it. It is complex, support isn't very widespread, and all the difficult questions are pushed to a "bandwidth broker" which requires too much knowledge of the network topology.
SDN gives us another approach. Set up the IETF Differentiated Services in the core. At the edge do admission control into that DiffServ core. But how to do the admission control when there so little support for IntServ?
The obvious thing to do is for the SIP entity which SIP-routes the call to send a note to the OpenFlow controller, which then puts some rules on the edge switch above the customer to admit the flow into the QoS class (at a minimum: for the specified IP address and UDP port, rewrite the DSCP to EF).
Yeah, all of this is an obvious application of SDNs. But in this age of software patents, that's well worth saying.