That's all well and good, but if you are constantly the 5th or 6th man due to you being the next cab off the rank, the player themselves will suffer due to possibly not getting any game time. Multiply that by the 17 teams in the NRL and that's a lot of players potentially missing out on showing their ability.
I think halving the amount of players by limiting it to 5 instead of 6 is the best way forward. Or even require the player to get rotated like the 18th man does.