A few years ago my commute took me past a semi-truck wash where they hosed out all the trucks that had just unloaded at the nearby stock yards. A lot of what they washed out of the trucks floated out into the street. Pretty sure that wasn't mud I was riding through every day.

I have two bikes with fenders, and the rest are without. If rain is in the forecast I'll ride a bike with fenders. While I prefer the non-fender look, I don't prefer it more than getting soaked from all the moisture coming up off the road. They may or may not help with keeping gunk off my drivetrain, but they undoubtedly help with keeping the gunk off me.