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.
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.
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