Small bump to this thread now. I've been simracing since 2004, but never have I, in a serious simracing event, raced in rain. All championships I've done have been dry weather, all club races I've done have been dry weather.
Last Saturday I did a 9hr endurance event at Paul Ricard in AC:C, with dynamic weather and a less than 50% chance for rain. When we jumped to the race session it suddenly rained. We decided to go for Wet Preset setup (we hadn't tested in wet at all), and hope that it would rain for at least 5 hours.
As it turned out, it was raining for the whole 9 hours, and I must admit, it's one of the most fun experiences I've ever had in simracing. I really loved racing in the rain, and I hope this wasn't the last time!