You didn't link to any specific product but from what I can find by googling "650d silicone case" those products provide no water/dust protection whatsoever.
The big opening in the camera that let dust/water in are, in order of importance:
The lens mount
The battery door
Various connectors (USB etc.)
Buttons and dials
Any case that only covers some of the buttons is absolutely worthless for water/dust protection.
The silicon cases may provide some very small level of protection if you drop your camera, but considering that DSLRs are pretty rugged and the cases don't protect the biggest weak spot of the camera (the lens/ lens mount) I believe they are probably completely useless.