In previous times with 25 minute turnarounds, lower fuel price and resultant max speed cruise our aircraft would regularly overspeed into the clacker for example when exiting a jet stream. Our safety pilot and the fuel price have pretty much stopped this. But there are clearly situations the autothrottle cannot cope with.
As far as the seatbelts are concerned I find at least in the short term it makes things worse. It seems to have a pavlovian effect of encouraging people to get up and go to the toilet because they think otherwise they will be stuck seated for a while...not ideal when encountering unexpected turblence.