If you know your enemy is going to come to you and give you a terrain advantage, why would you go to them? I don't think Onya can ever or almost ever kill Rogran outside the Spirit, but Rogran can definitely kill Onya outside the spirit, and I don't think Onya can die at the Spirit, and he can definitely kill Rogran there. Why take a battle where you can die, and you can't possibly kill the enemy, without cabal items or some other reason to fight?
Also, I don't think you realize how good bloody shackles is. Having been on both sides combined 100+ times, it is a) really good b) crushes mages and c) can be very unpredictable on how hard to survive it makes life. The second Onya gets bloody shackles on him, it goes from being a PK to a "how can I survive this" fest.