It’s more annoying than anything. There usually isn’t any direct harm to me or other drivers, though there’s always the chance of a butt landing in an open sunroof or window, or igniting roadside grass or leaves during particularly dry times.
You often can anticipate when a butt will fly. Look for the cars driving with their windows half-down, regardless of whether it’s 100 degrees, 10 degrees or raining buckets. First you’ll see the occasional puff of smoke. Then you’ll see the hand, trying to appear nonchalant, drop or flick the butt away.
I understand why ashtrays have disappeared from vehicles. Fewer people are smoking, and ashtrays weren’t the prettiest items. Carmakers used the space to add more popular amenities such as charging ports for electronics and better entertainment systems. I bet an ashtray still could be squeezed in and disguised to look nicer than they used to.