On my older bikes, my rear racks (various brands) were not enough to prevent me getting a little grimy. So, I added a fender to prevent that. The added benefit was that my front derailleurs didn't get packed with dirt.

On my newest bike (GF 2011 Trek Montare), however, I had to cut away bits on the Planet Bike Freddy Fender to get it to work w/the derailleur and Planet Bike Eco Rack. My clothes still stay clean, but my components, not so much. I probably could have spent more on a rack that would have had an adjustable height preventing that, but I couldn't find one that would hold my U lock in the slot w/o having to be locked in place.