I'm somewhat in agreement. The extra cloth making the suspenders on bibs add weight and heat. The only option for natural breaks is through a leg or remove jersey first. Takes longer to don than shorts. It seems a poor tradeoff for not having an elastic band at the waist.
It isn't as though cycling shorts have trouble staying up. There are not really any forces that pull them down. The saddle forces the material up from the saddle down, so the waistband only has to hold up the material from the saddle up.
Like you, I'm not sure of the bibs advantage. I have both bibs and shorts. To me, the longer the ride, the greater the advantage of shorts. This means bibs have a greater advantage on short rides, whatever this advantage is.