I second what LessO2 said; our pilots are probably quicker to turn on and later to turn off the signs than foreign carriers due to the fear of litigation should someone get injured when the sign has been turned off or not turned on.

That said, this engenders pax who feel, "Well, it's really not that bad, and seems like it should have been turned off a long time ago, and I really, really, have to go!" They disobey the signs because they don't feel they are justifiably on ... it's like the little boy who cried wolf. If you overuse them for the lightest chop, people will fail to heed them in a case of really needing to stay seated.

I've had my head knocked pretty hard while in my seat with my seatbelt fastened before, so I certainly don't doubt the need for pax to stay seated for turbulence... but I do admit to having questioned the sign on many an occasion, too.