I've gotten poured on a couple of times. It sucks. but the only time I was ever scared was when I got caught in very heavy fog on a two lane between Rockford, IL and Galena. I couldn't see more than about 20 feet in front of me on a modestly curvy road that goes through the hilliest part of IL (several 10% grades). I had to stop twice to wipe down my face shield. And that bike had a MUCH brighter tail light and headlight. Fortunately it only lasted for a few miles, I know the route well, and I was able to follow a semi and use his tail lights as reference points.
I'm working on borrowing a rain shell from a buddy (it says Harley Davidson, but hey, it's free) and I already have rain pants, waterproof boots and Gore-tex crab claw style glove covers that I use for bicyling. So rain or shine I will be good to go.