It is a bit confusing, but it significantly reduces delays when conditions are suitable - it means ATC can launch a departure with an aircraft on short final which saves a significant amount of time. You will notice the delays caused by non-circling aircraft particularly at WLG.
If conditions are above the limit, you are required to join the circuit, so it would make sense to set circuit alt on final, rather than have to quickly change it after a go-around
In practice, if you have to go-around due to runway sep etc joining the circuit gets you on the ground a lot sooner so surely that's a good thing. If you're going around due to weather conditions, it will always be the standard MA anyway.