If the seat belt sign was only about keeping your belt on while seated, it would not be a problem for me. The problem comes when you are expected to stay in your seat for hours. My doctor told me to make sure I got up once in a while to avoid "economy class syndrome". Good advice, except it's very difficult to do when the sign is on for the majority of a flight. Not to mention the need to use the rest room. The crew seems to forget that when you take off, you might already have been in your seat for an hour or more. Then the sign stays on for another hour or more. It varies from person to person, but some simply cannot wait that long to use the lav. As a result, you usually see passengers getting up while the sign is still on.
Anyone have any numbers as to the actual number of injuries per year caused by turbulence? I'm guessing that it's very low.