Do the shorts move that much out of place that bibs are necessary?

I've actually been more comfortable in my regular biking shorts than my bibs, but my bibs are a little too big.

Once you go bibs, you'll never go back.

Or at least I didn't.

I find that bibs hold your "cash and prizes" up out of the way a little better than shorts.

I also find that bibs, relative to shorts, lower the probability of showing off your butt crack to anyone.

I like bibs too, cause they hold in my beer gut.

for me, yes. I just made the mistake yesterday of doing a 60 mile ride in shorts that I rarely wear because all my bibs were dirty. The back falls down so much that the entire (gigantic, in this case) chamois gets pulled to the front, offering no protection to my sit bones. I stopped and hiked them up a few times, but it became futile very quickly.

Never had this problem with bibs.

Sometimes, yes. Also they're more comfortable than tying the waistband which can be constrictive and annoying.

I prefer the shorts. Bibs always make me feel like I'm wearing old man suspenders.

For me it was more about the lack of waistband, which on previous cycling shorts were in uncomfortable places.

I went bibs, and I often go back.

It is a matter of preference/choice/individual taste. As a matter of necessity, I think not. I prefer them myself as a matter of my comfort. YMMV

Necessary? Depends.
Should you buy a pair? Yes.

I don't like the waistband on shorts. I like how I can wear shorter jerseys with bibs and not have my lower back sticking out.

shorts on the trainer

bibs outside

Bibs for life.

The difference between bibs and shorts for any ride under 20 miles is irrelevant, in my opinion.

I just feel that my belly is too damn pasty for the world to see when I completely unzip my jersey.
I like to be considerate, lol

Unless you end up riding behind some overweight guy in shorts flashing ass crack. That has nothing to do with mileage.

I much prefer bibs. However, I occasionally wear some old shorts when using my trainer.

how'd you end up behind a guy that overweight??? are you that overweight and out of shape as well??

Obviously they are not necessary or everyone would always wear them. Many people prefer them. Ymmv.

I use both. I find Bibs are too warm in the heat - even the vented ones.

I prefer shorts than bibs. I went bibs, and I'll never go back!!!

not worth repeating...

True in my experience. I'm a late convert. Been riding since the 70s ut only tried bibs ~ 5 years ago.

Agreed.

and agreed

and agreed

But does sweat collect under the straps, even the vented ones?

I'm sorry, English is not my native language.

I prefer shorts. I have a pair of cheap Performance bibs and never wear them. The shoulder straps are uncomfortble to me and they are harder to deal with when I need to make a pit stop. I also find them hotter. My shorts do not move around. No crack is showing and the waist band doesn't bother me.