The rebound slide was intended more to retract (rebound) the hammer than to act as a hammer block. Retracting the hammer from the primer after firing is a necessity in a side swing revolver. While a top break will open with the firing pin buried in the primer of the last fired round, a side swing revolver won't open unless the hammer is retracted.
The early S&W hand ejectors (S&W's term for its side swing revolvers, in contrast to the "automatic ejection" of the top breaks) used a rebound lever, powered by a leaf spring. That was changed c. 1906 to the current rebound slide. But S&W concluded that the rebound slide, while it worked as intended, would not stop a blow on the hammer from firing a cartridge, fitted a hammer block c. 1915. The block went through two variations until c. 1945, when the current type was fitted. The previous blocks were spring loaded, which mean they would be ineffective if the spring broke or grease or dirt held the block out of the way. The current block is positive in both directions.
Jim