The bf had front and rear mudflaps on his ST and the sides of his car were always much much cleaner than mine after the same amount of driving, so I'd say they stop more than a little bit of mud.

I dont think they'd stop stones flying up and damaging the side stripes because both our cars are roughly the same age (2 and a bit years old) and both our sides stripes are fine. What you've got to worry about with stone chips is your bonnet, front bumper and wings.