Belts wouldn't really do that much to stop you from falling out, the force taken to break the three 'clicks' that your harness has had to pass would be more than strong enough to rip the belts out.
We were actually instructed on Nemesis to pull the seatbelt first, then, with both hands on the handles, push then pull the restraint to ensure it is locked.
Thinking back, the system can go wrong. On the nemesis trains, eah row has 3 strips of a brassy looking metal on the top right, which corrispond to three spring loaded 'switches' hidden inside the bar that has the 'put in your seatbelt' stickers. The 'switches' pass a current through a circuit that can only be complete if all four harnesses are locked down three 'clicks'. This current registers with the computer and is shown to the op by a light illuminating (Nemesis rows are actually grouped in twos on the ops panel).
On one wet day in 2006, the train was about to depart, when a host noticed that one of the seats on the back row wasn't closed, yet the light was illuminated and the ride was ready and authed for dispatch by the computer. We downloaded the train, and sure enough, the ride started to dispatch.....we worked out that somehow water must have got into the circuitry at the back of the seat and somehow 'bridged the gap' to create a complete circuit.
Needless to say the train was immediatly garaged and checked.
Weird though, eh?
Another thing that sometimes happens is the harness 'pops', which basically means it is not fully on a 'click' (or a tooth on the ratchet system) . On the first drop, when pressure is applied, it will pop to the next tooth, and people come back terrified thinking that the harness was going to open fully. This can mean the harness will dispatch with 3 'clicks' yet can return on 2.
And yeah rita, some rides can send without harnesses down can't they, like corky.