Since I haven't yet played, I can only give an educated guess. Rock Smash will not be neccessary to continueing through the main story of the game. However, there will likely be some areas where you can't go without Rock Smash, sort of like how Flash might not have been an HM in Gen IV but was useful for areas such as Dark Cave. Rock Smash will also likely allow you to find some certain Pokemon, like Shuckle could be easily found with Rock Smash in HG/SS. It also might be possible that items can be found using Rock Smash.