Dummys work just fine in a 3rd person rugby game.
Think of this, you've got two guys running at you, one with the ball... Your defending and your doing your best to keep them both under crontrol, keeping an eye on both and trying to work out if the ball carrier is going to pass, step, run right at you, try and run around you, rah rah rah. At the last second your going to have to make a choice, do you tackle the current ball carrier or do you take down the the guy outside (or perhaps inside, for that matter), because you've put the ball carrier under too much pressure and the time for an offload has come?
If the ball carrier does a dummy your instinct is (most of the time) to do some quick math and pull that defender onto the supporting runner. Seeing as it was just a dummy you've just put your lone defender onto a player that doesn't have the ball, and the other guy can sail thru for the try.
Dummys are very effective.
It's just like saying that a sidestep only works in 1st person.
I'm a special move supporting as you could probably gather