In the event you crash, you need to be able to assess the best way out of the plane. Doesn't help if you can't see what you could be jumping into.

Turbulence is rated as light, moderate, severe. No one can truely predict turbulence, so they go off of what other pilots are reporting. So who knows why some pilots are so forceful about it.

But a few years ago, on an American Airlines Airbus A300 from Boston to San Juan, most of the flight attendants were in the back galley trying to find a rat (seriously). They had carts pulled out, a few in the aisle, and everyone was doing what they could to find it and put it in a container or box until they landed in San Juan. It was a VERY smooth flight. Suddenly, the plane dropped about 3000 feet. Lets just say they weren't worried about the rat anymore. 2 FA's broke their backs (including my friend), 3 of them broke legs/ankles, and 2 others had concussions. They weren't even "legal" to land as only 1 was not severely injured. Fortunately they had a handful of non-reving FA's (and pilots) who helped out. I believe a total of 16 passengers had injuries that required at least an overnight in the hospital. From what my friend said (guy who broke his back), none of them were able to work again. The one lady who wasn't severely injured was flying, the plane bounced a bit, and she screamed at the top of her lungs. AA grounded her (with pay, of course).

So... I don't know about you, but if the seatbelt sign is on, I'll abide. If I REALLY need to get up, and when I do, I always keep a hand on the overhead bin railing (well, while the planes still have them) so I have something to grab on to if we hit some bumps (to this day, my peve, and what I don't get is why people grab seatbacks while walking up/down aisle).