Okay, so since I seem to be lacking in energy for new posts, I'm "borrowing" one I posted to the blog for my employer, Bikes@Vienna:
If you come by our shop, most days you'll see one of my bikes sitting out front, since I ride in most of the time. Folks that know me and know my bikes can pick them out by a couple of key features, one of which is the fact that most of my bikes have fenders on them. In this region, that sets my bikes apart from the vast majority of bikes, since folks here generally shy away from fenders. So why do I buck the prevailing style? It's not simply curmudgeonly behavior... there's a practical reason.
Take a look at my Bridgestone to the left there. Look closely at the tires and the inside of the fender and especially the black mudflap at the bottom. Notice the plethora of mud caked on the tire on the inside of the fender/mudflap? That's the accumulated goo from two recent rides on the C&O Canal after rainfall. What you don't see is any appreciable gunk on the rest of the bike, or on me, after the ride. In marked contrast, on both rides I saw a number of other folk riding bikes without fenders who had amazing mud streaks up their backs.
Aside from the forays on the towpath, one of the things I enjoy about having fenders on most of my bikes is that I never have to think about whether or not it's going to rain... I'm always prepared. It makes my bikes ready to go any time, any place... in other words, versatile. And honestly, they don't add significant weight or air resistance or anything like that. Sure, you probably wouldn't put them on a high end carbon racing bike (you probably can't fit them anyway), but on many other bikes they are a great feature.
I used to think in terms of having a "rain bike"... now I suppose I have a couple of "sun bikes"... the few without fenders that I only ride when I'm reasonably sure the weather is clear, while most of my bikes are fendered. It just makes it easier to have fun on my bikes. So give some thought to adding them to your bike... I don't think you'll regret it, and you may very well find it helps you get out on your bike more often, and that's always good!