yea it sucks when your on the lifts with gapers who are all anal about the bar, and i think its dumb to put them down on low speed chairs, but if your on a highspeed you should always put them down. a safety bar saved my life, or at least saved me from a ton of pain and a potential end to my skiing career.
last year late in the season, i was skiing at sugarbush in vermont, pretty much the only one on the mountain.  there was a lot of snow melting, and the power station flooded or something.  long story short the fuses at the mountain blew, bringing all of the chairs to a DEAD stop.
anyway, the chair proceeded to rock back and forth, getting above 90 degrees to the ground both ways, and swinging wildly.  every time the chair went back, i would get pushed against the safety bar, pretty much hanging on it.
i would have fallen out of the chair if i hadnt had the bar down, and fallen at least 35 feet to the ground.