Driving a sports car is not always simply about going "fast".  They are just as much about the feel and sound of the car, even if driving slowly.  Only issue is tires...R-comps aren't always that great in the rain (my Elise will hydroplane).  Oddly, my former 135i was a car I took out in the rain as much as possible because it was so much fun, but that car had Michelin PSS. Frankly, one can only just scratch the surface of performance of a GT3 on the street, and thus, you sure as hell better appreciate many other aspects of the car, otherwise it will collect dust.

Another point: as someone who spends a decent amount of time in Vancouver, B.C every year....I know that if you only drive your sports cars when it is not raining, you would cut the available days to drive in half.  Same thing for our friends in the U.K.

For practical purposes in Southern California, choosing to not drive in the rain is of no consequence because it rains infrequently.  I have driven my GT4 in the rain maybe twice since taking delivery almost two years ago, because the car is not a daily driver.