One problem is the F/As lose credibility when they're up and around with the seatbelt sign on. Turbulence could just as easily throw you onto a passenger, with both of you getting hurt. It's one thing to get up to do a seatbelt check walkthrough, but many stand in galleys and elsewhere talking even when the light is on. When passengers see that, they're more inclined to think it's ok to get up too.