First, if you can't see because of mist, all you have to do is use some Rain-X. Don't blame other drivers.
Second, mud flaps on semis are mandated to cover at least 50% of the tire's rear exposure. Most cars don't have enough exposed rear tire to warrant mud flaps.
Lastly, if you install mud flaps on a car that doesn't need them, expect them to rub against every speed bump you drive over. All the mud flaps I see on cars are bent backward from rubbing against speed bumps. You spend your money, you install the mud flaps, and then after a while, just as much tire is exposed as if you didn't have the flaps at all, making them a complete waste of money.
If I drove a pickup truck, I might consider installing them, but on passenger cars, they're useless. Look at the name "mud flaps." Do you drive in the mud? Do you drive off-road? I don't. If you were to ask me about water flaps, I might take a different position.
EDIT: As I was driving in the rain yesterday, it occurred to me that most SUVs probably need mudflaps, but most cars probably don't. It looked to me like most of the mist being kicked up came from SUVs and pickup trucks, not from passenger cars.