When I took the rider safety course, the instructor said our tires have 85% of their dry traction in the wet. That goes down obviously when you ride over paint strips, leaves, manhole covers, etc. I try to ride in the summer, avoiding those same hazards so that it becomes second nature in the wet. It takes a lot of the worry out of riding in the rain.
I have only a couple times when I don't want to ride in the rain. The first is when I have to get out of my rain gear in a parking lot. Going to the prison was like that. There's nowhere inside to leave my gear, so I end up taking it off and on next to the bike with the rain coming down. That sucks. But if I can take my gear off and on inside somewhere...no problem.
The other is riding in the rain in the dark. I don't mind riding at night. I don't mind riding in the rain. But when you put the two situations together, it isn't smart to ride when you have a car sitting in the driveway. The gray pavement turns black and sucks up all the light that hits it. The rain can end up getting on the inside of the visor...and making it only slightly better than a bathroom window to see through. Combine that with the pavement that sucks up all the light...and you could run over a 2x4 or tire carcass that the car in front of you simply ran over.
A couple years ago, I felt like the biggest hazard out there, was...wait for it...other motorcyclists. Or wanna-be's. I'd be minding my own business, and some guy would do some stupid riding just to catch up to me and ride right off my rear tire. I wanted to tell them that if I wanted a group ride, I'd invite you...so back off. And a few even decided to pass me in my lane with no warning at all, like it was some great game. But! ...when it rains, those jerks are off the road. If I see another rider in January or February...I can pretty much assume they are pretty professional riders, or they wouldn't be out there.
Cars don't bother me at all. I have enough reflective tape on the bike to make it really stand out in low-light low-visibility conditions. I know this varies by region, but around here there are so many drivers who pay attention to motorcyclists...and will give me plenty of room. It's like they just "know" that I'll do something stupid right in front of them and crash...and they don't want to run over someone. When I drive the same roads, it is like I'm just another pylon on the race to work.
Chris