... cannot expect a player to stop on a dime if the defending side are not competing properly.
The point has been made that if the defending side doesn't commit any players at all to the ruck, then it is not, in fact, a ruck at all making it legal for players to come around and stand in the attacking backline!
I'd say it would have to be a brave team to try this and the world cup is probably not the best place to test it. For now teams will just have to respect possession more.
The first time I saw something like that was indeed in a RWC - I remember a Samoa tactic used in a couple of 2007 games (one I think was not surprisingly against England) to neutralise the rolling maul (usually where a rolling maul was predictable such as from a lineout, or from almost any English set-piece).
As the 'attacking' dumb juggernaut was getting itself set-up, the Samoan pack would disengage and one would run around the back to challenge for the ball directly.
It worked well enough the times that I saw it.
This year Ireland may bring a comparable counter-intuitive tactic to the fore - the
choke tackle favoured by Les Kiss their defence coach. The idea is that rather than bring the attacker down, the tackler wraps him up, usually with help from a 2nd defender. Once a few players join in the ref calls a maul and if the ball is not recycled by the attackers, then a scrum is awarded to the defending side.
The risk is that the upright body positon required to defend in this way does not halt 95kg centres coming at speed. The attacker could break the tackle or drive on 10m-15m.
That said, it has worked at various times in games for Ireland and is one way of generating turnover ball.
It is preferable to my pet hate of endless kicking ping-pong while both sides hope the other one drops the ball.
However, I think the current vogue for trying to rip a ball out of the grasp of a ball-carrier as he is tackled is more visually-engaging and skillful.
Cheers
Megweya