Dawn and I were flying from Munich to Amsterdam on a KLM CityHopper Fokker 70. Very shortly after takeoff, the flight attendant announced that the flight crew would be soon turning off the seat belt sign, but KLM encourages passengers to use caution in deciding when to unbuckle their seat belts. Sure enough, the seat belt sign came off around 10,000 feet, and stayed off until the descent through around 10,000 feet. In cruise we encountered pretty decent chop several times, prompting the crew to change altitude - but the seat belt sign remained off. It was truely left up to the passengers.
This is quite different from anything I've seen in the states. There are several pros and cons to this different philosophy, but I'm not sure that safety is any different than what we have now.
The biggest pro from a flight crew standpoint is that under the KLM policy, you would not be held responsible for any passenger injuries. Here, if a passenger is injured while the seat belt sign is off, your actions are going to be quite scrutinized to see if there was any reason you should've suspected turbulence: Did you look at the Airmets? PIREPs? Did ATC pass along any ride reports? Didn't you know about that bend in the jetstream? Etc. Under the KLM policy, the decision is out of the flight crews' hands and into the passengers'.
But isn't the flight crew better qualified to judge if it's safe? you ask. Well, yeah. The passenger surely isn't reading Airmets or PIREPs, and they don't even have a view out of the front window. That said, the truely dangerous turbulence falls into two categories: big lumpy stormy clouds, and severe clear air turbulence. In the former case, it'd be pretty obvious to passengers to stay seated; in the latter case, even flight crew won't know about the existence of severe CAT, so even in the US many CAT incidents occur with the seatbelt sign off. In cases where CAT was predicted or reported, I think even KLM crews would turn the seatbelt sign on.
To my thinking, the safest option is a flight crew that continuously monitors the turbulence situation and leaves the seat belt sign off whenever it is less than moderate, coupled with flight attendants that enforce compliance with the seat belt sign. Like I previously wrote, most flight crews are considerably more conservative than that. That's great for protecting your licenses and job, but I don't think it increases safety. When the seat belt sign is unnecessarily on, it might as well be off for good, because you end up with similar numbers of passengers out of their seats. In this case, better to go the KLM route and remove liability from the flight crew altogether and allow people to use the lav without breaking the rules. Of course, in the United States' litigational environment, I'm not sure that this policy would survive the first lawsuit. "Whatayamean, personal responsibility!?"