I rode after it rained this week and one corner had a lot of mud that had then been tracked up the street by car tires. I took that very carefully -- I could imagine how slick it was!
My only wreck in 2 years and 3,200 miles of riding... was on a road where it had just drizzled... and there was layer of mud on the road... It was so slick.
The moment anyone hit their brakes... BAM!... down we went. Actually out of 12 people, 6 people went down.
And even though I was only going 10-14 mph, it hurt LIKE HELL. My shoulder took the brunt of the fall and 2 months later, it still hurts. I train for tri's and I had to stop swimming for a month or so to let it heal up. And, I still can't breathe on my left side (too much pain in my shoulder, but luckily I only breathe on that side when fighting waves hitting my right side).
So, for me... it's NOT WORTH it to ride when it's raining. I would rather keep my body and bike safe. Not worth the risk. Now, if I get caught in rain when riding... that is a different story... but even then I try to stop riding until it clears up.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"