I ride year 'round, and I leave my fenders on year 'round. During the summer months, I'll take the mudflaps off when the weather clears up for weeks at a time. A little splash of summer rain isn't going to ruin anything.

During the other 3 seasons, it's full fenders with full mudflaps. The fall is rainy and I don't like getting a constant stream of road grak sprayed on my drivetrain and my water bottles. The winter and spring are rainy/snowy/slushy and we don't use road salt around here; it's all cinders and sand. That combo is hell on a drivetrain, so I keep it contained to my fender wells instead of spraying it all over. The front mudflap is mostly to keep me and my bike clean. The rear one is a courtesy to people I ride with so I'm not sending up a giant rooster tail of grime into someone's face.