Either a roof on 1 or 2 courts, or lights for night play. During a 2-week Slam, there is bound to be rain, from insignficant amounts that annoy but don't get the tourney off track, to huge amounts that wreak havoc with the scedule.
The US Open can get away with no roof, because night play opens up a huge amount of make-up time if it rains (even the 2003 USO, which was inundated for most of the tourney finished on time). They'll start matches late under the lights, and during the first week will play outer-court matches at night if they have to. This could be done at Roland Garros instead of a roof.
Rain and delay is part of the sport and I don't necessarily think a roof is absolutely necessary. But, given the reality of huge television contracts and a viewing audience who wants reliable, live tennis (instead of a billion showings of Federer/Querry), I think a roof on at least one court much more of a positive than a negative.
Remember, if the forcast is for a short period of rain followed by clearing, then the roof doesn't even have to be used. They can wait out the rain. But, it is really helpful when there is no hope for clear skies over a long period.