1. They stay in place.
2. See #1, because it's a lot more important than people realize.

The whole point of cycling shorts is to keep a layer of fabric as close to the skin as possible, and as much of the time as possible. But, even good shorts have the chance to sag, letting the nose of the saddle hook the chamois and the crotch to get loose and cause chafing.

Bibs don't have that problem.

The extra step required to poop is hardly a con because you're going to have both privacy and time to do what you need no matter what you're wearing. That is, unless you're in a speed-pooping contest, which I've never heard of.

Provided they fit, and there are some brands for extra-tall torsos, too, there aren't any real reasons to not wear bibs.