In some systems (including the real world) it is much easier to destroy than to build.

In a player shaped world you need to prevent making it easy for players to undo what other players have done.(it may not even be a case of the usual mentally ill griefers but just someone else who has their OWN plans for the world assets involved)

You also want some persistance of the 'shaping' so the player can have a chance to see it as 'done' before the world moves on and his efforts are modified by others actions.

I can wholeheartedly agree with this.