Depends on whether you're watching at home or not. Racing in the rain is generally much more fun for me to watch as you see people spinning off the track all the time.
Then again I prefer watching races other than Formula One where a car can't be taken out by an insect hitting the windshield.
Well Formula One and IndyCar are slightly different than, say, stock car racing (NASCAR); the tires are designed so that they don't get any real traction, but just so that they go zoom really fast. Basically, 1/16th of an inch of water on the track could cause a serious accident. I love open-wheel racing, simply because my fondest sporting memory ever was watching that amazing 500 last year.
As for the accidents, some pretty terrific accidents can happen in the sunshine too; again, I refer to the 2006 500 incident between Castroneves and Unser Jr. (I believe) or the Thomas Scheckter wipeout into the pit barrier. Hell, even the last accident at the most recent Daytona 500 was pretty spectacular, with damn near six cars being wiped out of the race. That's the real reason you watch racing (besides hoping your team/car wins, of course

).
However, I will agree on playing certain sports in the rain. Football (American and European) is sometimes best played on a mud-soaked field in a downpour, as that's exactly what I used to do when I was a kid (read: 3 or more years younger).
