Frad, the stand rule was brought in because the league felt that the way players were aggressively manning the mark, it wasn’t enough of a reward for the team that took the mark or won the free kick. It was also hard for the umps to officiate whether something should be 50m when players were manning up diagonally on the arc (ie cutting off the middle). There were too many angles involved.
Clubs like Collingwood started stretching the stand rule by sending players up to the ‘stand’ point, then backing off. Basically it meant they could pressure the player who took the mark to go back behind their mark, but then run backwards where they could move and defend other players again. Having players be able to swap who is on the mark would do exactly the same thing.
The AFL has made the rule clear, you either come up to the mark and stand, or you get out of the protected zone. If you change your mind halfway it’s a 50m penalty.
Walker went to stand the mark, then Freo decided someone else should stand the mark. It was a clear penalty. It might not have been all Walker's fault - Treacy could have called him away - but it was his action that caused the 50.
But as to the rule, go back and watch the game – or any game – and see how many quick play-ons and short handballs are given through the middle because the defending player is a statue. It’s really opened up that sort of play, and there’s much fewer instances where defending players are cribbing over the mark and the ump has to decide if it’s too much.
It’s a good rule.