I get to ride in the rain quite a bit this time of year. It's fine with the right clothes, traffic and light conditions.
Riding on I-5 during rush hour in the dark in the rain is right at the limit of my risk calculation strategy though. I would not do that on a regular basis. There's too much chance for someone in a car to stop short or pull into the car pool lane while your zipping by - and no matter how good a rider you are, you aren't going to be able to slow down in time. When traffic is heavy the car spray can cause a fog, you deal with the lights of oncoming traffic causing glare on the rain drops on your shield..etc. Once you get above 50 or so the wind will pretty much clear your face shield - certainly good enough for daylight, but not so much in the dark.
The main thing is to ride smoothly, slow down, ride at a pace that you are comfortable with, if possible stay off the road right after a light rain (when the oil and stuff hasn't been washed off), watch your following distance, plan ahead/look ahead, don't stop/start quick on the white slippery markings at intersections, and practice in low traffic conditions so you get a feel for the traction.
If I was planning a trip where I might be riding in the rain/cold, hot, or even a little snow I'd try and practice that riding and make sure my gear is adequate, on short hops first. If snow was sticking to the road or it's icy - I'd pull over no matter what. When I rode up from San Luis Obispo a couple of years ago in February I'd been using my gear around town and had practice riding in the rain - but I'd never been on that bike, and I've ridden when there was a little ice and snow on the road - so I don't always follow my own advice, but as a general goal it sounds good to me.
I was accelerating onto I-5 this morning going 55-60 mph - in the dark in the rain, and gunned it just a little too much - the rear tire started to spin up and I got a little weave going. Live and learn. I like the challenge/focus of that - you don't get a lot of free passes to learn so you ride with care, smoothly, taking into account your abilities and the bike. It seems very elemental? to me. It's you and the elements - the cars, rain, road - and your concentrating on surviving some times - enjoying the ride most times. It can be very relaxing in an odd way since you are going to forget about whatever else was on your mind as you focus on what's happening in the moment.
Which brings up one final point - if you find you can't focus for whatever reason, you're very tired, really stressed out to the point of not being able to let it go, intoxicated, too cold or hot, - it's best to stay off a bike. I know from experience you can survive doing that for short periods of time but over the long haul it's asking too much of your guardian angels/luck/karma or whatever you like to call it.
