Beyond preventing idiots from falling out of chairlifts from horsing around, has anyone actually felt the bar helped them in any way? In 28 years of skiing I can't recall a single time I felt the bar helped me stay in the chair due to swinging or sudden stops. It always struck me as more of a peace of mind thing for those scared of heights.
Several years ago at my home area (Mt. Hood Meadows -- where if it's snowing it's BLOWING!) high winds blew a tree down across the lift. No derailment occurred, but it did bounce the cable far enough that somebody without the bar down got ejected and broke their femur.
I know it's a "lightning never strikes twice" kind of thing, but ever since then, I've been in the bar down camp. There just isn't any downside.