SDN are service oriented networks, and if I get it correctly, are intended to make life easier for corporations. The overwhelming complexity of different network hardware and it's special config increases management costs. SDN can help to bring complexity and as such costs down (my understanding). And yes, it can increase security (down to the level of allowing only a single port or address).
Now our crypto ccy world, or bitcoin: we are talking P2P networks over the internet. No corporations defining it (hopefully, if not some NY hotel room agreements want to take over bitcoin). What could a SDN do? Isn't P2P already a way of defining a type of network, that needs nearly "no management"? And the structure of the bitcoin network is designed in a similiar manner. It provides one specific use case, that can hardly be controlled by a single corporation/government/user...
It becomes different for crypto ccy exchanges though. You can imagine SDN for trading, backoffice and payment (channels). But that is again company specific... That is probably out of scope for our "public" environment.
By saying this, I'd be happy if a discussion can start with proposals or use cases in our crypto ccy world. SDN is a fairly new approach, and crypto ccy is a new model. Looking forward to see potential benefits of their marriage