OK, I'm convinced. Bibs are useful. Until about 40-km rides I was quite comfortable with cycling pants, but starting from 50+ km I see that the waist rubber band really digs in, and when hunched over, one's stomach naturally protrudes. The pressure becomes uncomfortable.
If bibs have one advantage, it's that they rest on the shoulders and put no pressure on the waist. Maybe there are other advantages (no slipping—but the strong rubber band was doing a good job of that—what else?), but releasing waist pressure for longer rides is enough of a reason for me.
But if comfort is what it's about, what's with the insistence that bibs be worn on the skin underneath a shirt—for men and women alike. Wouldn't it be nice to distribute the pressure by wearing the bib over the shirt?
Is it that those who want to distribute the strap pressure need to wear an "underwear" no-sleeve shirt, then the bib, then a shirt? Wearing two layers on top would be unpleasant when the temperature is +30C.
(The keywords to use while searching for such a garment seem to be "cycling sleeveless base layer", or just "sleeveless base layer".)
Which leaves elegance as the likely reason. The cyclist looks a lot cooler (in the sense of 'hip') when no suspenders are to be seen. I agree. It'd even be a bit of a sore sight to see the suspenders. But elegance be damned. What if I just want to be super comfortable and have no intention of being photographed that day (poor you, Harrison Ford, you can't cycle in peace—they have to hound you). What if I'll ignore the looks coming from the cycling buffs who will no doubt look towards me with amusement, or with downright contempt.
Doesn't wearing bibs over—rather than under—a jersey make more sense, in terms of convenience and comfort for the rider—disregarding that it's inherently less elegant?
For the purpose of this question, we will assume that one has found a shirt that can itself be conveniently and comfortably worn against the skin.
In case it makes a difference, I'm talking specifically about road cycling.
One of the main unstated premises to this question is the ability to wear the same cycling pants or bibs two or three times before washing. This means that the rider does contradict the advice of bicycling magazines and does wear underwear under the pants or the bibs (it's quite all right if you disagree; but that makes your context fit for a different question). Merino base layers for winter sports can famously be worn several days in a row on a trip without washing, because wool is rather inhospitable to bacteria, which are what cause the foul smell. The lycra and other synthetic materials used for cycling pants and bibs are also a bit resistant to bacterial growth. Hence two rides in the same pants/bibs (travel; spend the night; return) are quite all right. Carrying extra underwear is easy. Perhaps you see now that this scenario provides a more compelling use case for wearing a suitable cycling jersey under the bibs, and also carry along a change of jersey. If the bibs do not fit too tightly, that's a recipe for poor aerodynamics from the sail-like bibs—as Criggie suggested—but on a long trip a bit of air inside tight-fitting bibs might be welcome anyway, even if it drops one's speed (we are hardly talking about racing here). The question remains whether some riding requirements make it compelling enough to wear bibs over a single jersey.