My attitude about leaving weights on the bar is simple - if you're strong enough to put them on, then you're strong enough to put them away. I have no time for the little fucktards that just get up and leave.
Maybe it's not too bad on something like legpress if someone leaves one or even two plates on either side, because you tend to warm up with that anyway, but for any other piece of equipment, even bench press, it's just bad gym etiquette. Maybe it's just my OCD coming out, but I like my training area to be as free of clutter as possible before I start. Everything has to have it's place!