You are unlikely during light chop to have a ground vehicle pull in front of your airplane making you either swerve or stop abruptly. Again, safety of all passengers. But that isn't the point. I've had several people get up to use the bathroom even during a short taxi. They are intentionally disregarding the seat belt sign which is on for everyone's safety, not just theirs, and need to understand that they are now delaying the departure. It isn't hard to hold it for the 30-40 minutes from boarding to take-off.