I used to wear them in hot conditions just because I had cheaper bibs with straps that were uncomfortable and could rub. I stopped when I got better bibs, as I found a baselayer to be the exact wrong thing for my riding. I'm generating the most sweat and overheating when riding slower, i.e., uphill, and a layer soaking up warm sweat is the last thing I want between skin and jersey. Then once I'm descending and not generating heat anymore, the damp layer starts evaporating and overchilling me.

A good form-fitting jersey that is breathable and can both wick and allow sweat to evaporate quickly is far superior. My Rapha Pro Race Aero jersey surprised the heck out of my with how cold I felt on my first ride wearing it on a cool morning, and it isn't even a true lightweight or flyweight summer jersey.

I reserve my old "hot weather" baselayers for milder conditions (50-60F) or when I'm riding indoors sans jersey.