I use fenders all year in MN. Obviously winter is more beneficial than the rest of the year, however there is a lot of crap on the road that is not water that will splash up at you or is water mixed with who knows what.

Sometimes the city empties a fire hydrant and there is water everywhere and I have to ride thru it. Sometimes there is sand on the road and my tires pick it up and flick it, it all goes inside my fenders not on me or my bike.