I think it depends on your situation. If you have a lot of data transfer without iSCSI Target in your system the transfer could be doubled. If you additionally configure the replication - you will get data transfer 3 times higher as before. So in many situation it is better to have it separated not only by simple subnetwork configuration - but by separated network hardware.

But if you will have situation that simple documents are copied over network from time to time - I don't think separated hardware is really needed. You will have only additional cost and more time spend on fixing problems if some network hardware will get broken.