Completely necessary. It's not just the rainwater washing over the roadway, it's also about the puddles. OR lakes, or rivers of water. The volume of water that a tire displaces up and into one's face, and into one's shoes, and soaking through every layer you have on (even waterproof) is truly amazing. And the stuff that's not water? Grit, sand, rocks, bits of pavement, oil, diesel, organic debris...in your mouth? All over your bike? Seriously, fenders rock. Even when it's dry I have bits of stuff all over my legs with fenders.