iSCSI Port Binding is the function that you use to bind multiple vmkernel Interfaces to the iSCSI Initiator to use Multipathing. But sometimes you don’t need iSCSI Port Binding.
If you have a single subnet for communication to the SAN you should use iSCSI Port Binding. Because the ESXi Host can only use one physical NIC to communicate with the SAN if you have multiple NICS.
So we have only a single connection. With iSCSI Port Binding the ESXi take both vmkernel to communicate.
But if you have multiple vmkernel Interfaces in different Subnet and the Storage SPs are in different Subnets you don’t need iSCSI Port Binding and you shouldn’t use it. The vmkernel make multiple Connection for each Subnet and there no longer is a need for Port Binding.
Here the KB Site kb2038869