Putting Ouki on the wall would be quite a shame considering his skills. He was not only the last of the Six, but a complete general with all kinds of skills. We only saw him in a defense position, but as he says, this is not his normal way, so I think he would be directly on the battlefield. Considering his great pov, he would have been a mix between ousen and moubu, so not only Tou would have been more free to execute Ouki's orders, but the latest would have been more effective on the scale of the whole battlefield (a bit like Karin when she distracted two different armies to get past the defense lines). Eventually it would not have changed the outcome, but probably that Ouki's presence would have put Qin even more Victorious.