Short rear fenders will keep mud off your back. Short front fenders will keep mud off your face. However, full fenders with appropriately long mudflaps, will indeed keep you and your bike mostly clean. Personally, I hate being grimy, hate having wet feet, and hate cleaning gunk out of my drivetrain, so I ride with full fenders on all my bikes. In summer, I'll take the fenders off one of them so I can feel like a roadie again, but I still leave them on the others for those inevitable rainy days.