[acallahan]

Maybe it's safer /not/ to wear a seatbelt.

It was hard to find good stats with quick googling, but this old source[1] suggests that blood clots during flights are the greater risk, killing at least 2,000 people in the UK per year. I suspect that keeping a seatbelt off while cruising lets you shift around more mid flight and would give a small decrease in the frequency of clots.

[1] http://news.bbc.co.uk/2/hi/health/1109406.stm

[lutorm]

You don't need to wear it tight (like you do with a car seat belt), we're not talking about tens of G accelerations like in a car crash. All you need is for it to prevent you from flying up out of your seat.