I commute in the rain; all the way from 80F+ to freezing. My bike only has a fender in the front; I rely on the bike's rack to act as a fender to keep my back from getting sprayed.
I don't care about grime and stuff on the bike since I know I'm going to clean it anyway; I just ride my bike like I intended to when I paid for it.
For me the keys about riding in the rain are:
* lose the fear of water; you're gonna get wet
* it's all in the breathability of the layers
* if you try to stay dry by using a rainproof layer, you'll sweat like a hog
* I'd rather be wet from rain than wet from sweat
* BUT, watch out and stay warm! (especially if you're riding long and it's below 50F)
* VISIBILITY, VISIBILITY, VISIBILITY; it never hurts to be TOO visible... (bright reflective clothing, head/tail lights, etc)
* anticipate and be less agressive, especially during turns
* to keep water from getting into my eyes and hampering my visibilty I wear sky/snowboard goggles
* to keep my feet dry and warm I wear booties (if I have them with me; weather.com is your friend)
* keep your clothes and other gear dry by using water-proof bags or panniers (Novara works for me).
* my bike has disc brakes; great stopping power even in the rain (but don't jam on them or you'll skid big time)
Riding in the rain can be great fun if done properly and exercising caution.
GreenAnvil