Nole said after his match that the roof makes things more humid, which has two main effects: 1st, players have to adjust their footwork, as it's more slippery (esp in week 1, i assume), and 2nd, the humidity makes the balls puff up and become heavier, so I would imagine this would help a big, flat hitter (like rosol) but neutralize spin? wawrinka, for example, hits flat usually.
Thanks for that info. Maybe now I'm hoping they don't need the roof! However, if the match is really late then I hope they have the sense to close the roof before it starts and not interrupt it, as at least 40 minutes is required - 10 to close it and a further 30 to ensure there's no condensation on the grass due to the air-conditioning.