Ok, as an old fart I'll weigh in here.

If you're a 120lb with flat abs, it probably makes less of a difference. But as you get a bit of softer living behind you, you may as many of us do develop not so flat a stomach. In that case and combined with the posture on a road bike shorts will have a tendency to roll down. Don't know about you, but I find it annoying. Bibs pretty much eliminate that.

Drawback is that in order to go to the bathroom you have to about undress, but to me it's a small price to pay.