Originally Posted by Western Flyer

The added wind resistance slows touring cyclists down a bit so I don't look quite as slow and I can draft them on rainy days without getting water and mud sprayed in my face. So yes, everyone should keep their fenders on.

Originally Posted by Jan Heine

Well-mounted metal fenders do not affect the bike’s aerodynamics. The front section of each fender shields the tire and reduces the wind resistance, while the rear fender increases the aerodynamic drag. The two effects cancel each other.