There are a couple of different attitudes to racing, and the answer to the rain question lies in which one you subscribe to.
If you are one of the guys that goes out there thrashing your car around every single minute of every practice session, not because you need the practice, but because hey - it is TRACK TIME, then I'd expect you to also be out there in the rain, although whether you'd be enjoying yuourself as much as you do in the dry is another question.
If you are one of the guys that builds his own engines and knows that every engine only has so many revolutions in it, and you can spend them racing or you can spend them touring the track, I'd expect that you, like me, would go out for 2 or 3 laps to make sure everything (including the driver) was working properly and then park it until the race.
I think that the second category of racer is less likely to enjoy thrashing about in the rain, and more likely to park it when it gets really wet.
The only times I would run rainy races was when I needed to for points, when I was running regular racing, not vintage. It just isn't any fun to run in the rain (in either my open MGA or my closed TVR).