How important is a rebuttal for a top conference (CS)?
First, it's worth noting that very few computer science conferences actually allow a rebuttal: mostly, the decision you get is the final decision.  In those rare cases that I have encountered that have a rebuttal period, it mainly serves to help disambiguate papers that are near the borderline.  Thus, it can certainly shift the status of a paper between accept and reject (e.g., by showing that reviewer #3 really was nuts and should be discounted or by eliminating the doubt that the authors were benefiting from), but is unlikely to do so unless the paper was already quite close to the boundary.