Just proves it's not a help having the week off.

It doesn't matter.

It get's done to death by the sky commentators but it goes either way each season.

It's helped 4 of the 8 Grand Finalists, so 50% isn't really 'proof'.
Teams to have failed....
2010 - Leeds lost to Wigan......Wigan finished 1st to Leeds' 4th so were still the 'better' team going into the game.
2011 - Wire lost to Leeds.......very experienced GF team against one without any at that time.
2012 - Wigan lost to Leeds......didn't help them missing their Man Of Steel nominated Full Back.
2012 - Saints lost to Wire......Wire finished 2nd to Saints' 3rd so again were still the 'better' team going into the game.

Swings and roundabouts though.

Extra game risks player injuries/suspensions and the fact you might lose, possibly a little fatigue

Week off might leave you a little under done and cold, doesn't give you any more preparation time because you don't know who'll you'll play in that week off.