I have the airoware outlyne vest. I use it every time I get on a horse now, after watching a friend hit the ground and not be able to get back up.
Would a vest be beneficial? I think it would be beneficial for most people most of the time. Horse riding is risky, anything that you can do to minimize the risk could be considered beneficial.
I have had one horse flip on me, but got lucky that I slid back after I hit the ground, the helmet /would have been/ the real help but I wasn't wearing one then... I don't think a vest would have helped too much, but I don't think it would have hurt me more either.
I do not consider my vest hot, but I have some tolerance for heat. I could be out riding in 80, 90F with a vest and feel warm but not hot. The foam ones do get warm. I guess it would depend on what you do. I wear a long sleeve shirt outside, especially when it's a hot sunny day.
It does restrict movement, but I find it to be minimal. I haven't found my movements impeded in training or riding, but it's a little harder to keep my elbows at my side and not looking like a chicken.
I guess it weighs a few pounds, but I don't notice once I'm riding.
I like the advice about the bull riders, they get gored, and probably have some protection to help minimize that risk.
Of course it is entirely your decision, but if you can afford it I don't see the downfalls to using a safety vest!