I'll refer you to the late Sheldon Brown's page on fenders -
http://www.sheldonbrown.com/fenders.html.
Here's some relevant quotes from that pge:
"If you are a "fair weather" cyclist, you don't need fenders, but if you are a serious cyclist, and don't live in a desert climate, you really should have at least one bicycle with fenders.
Fenders by themselves won't keep you dry in a pounding rain, but they make a tremendous difference when you are riding roads that are wet from drizzle, recent rain, or snow-melt.
Even in hard rain, you will become wet with clean rain from above, but your body and bicycle will be protected from the mud and sand kicked up from dirty puddles and rivulets."
and,
"Many cyclists protect themselves from rain by wearing rubber clothing, but they forget that their bicycles don't like dirty water any better than their bodies do.
The water kicked up by your wheels is much worse for your bicycle than the clean rain falling from the sky. If you ride in wet conditions without fenders your chain, derailers and brakes will all get sprayed with sandy, muddy, scummy water, often mixed with gasolene residue. This is very bad for these parts.
Even more vulnerable is the lower section of your headset. Headsets are designed to shed water like the shingles of a roof, and are basically rainproof...but the gritty spray from below has easy entry to the bearing surfaces of the heavily-loaded lower races."
Exactly the reasons why you need
Full Fenders (not those sh*tty mudguard style fenders, those shouldn't be called fenders at all), your parts last longer and you won't have to replace your bottom bracket every year like Pcad does. Not to mention it's easier to clean your bike afterwards should you choose to do so.