In case obfsproxy is running in a vpn node, then obfsproxy.gateway_address will be openvpn.gateway_address. That's my understanding about the evaluation of the above expression. Now, i don't understand why you think it's not used. It is used, since obfsproxy daemon has to know where to forward the traffic (in our case it must be a vpn gateway, even if they are in the same machine/IP).