Here's my take. Rain is SoCal is rare and usually light (I'm a Chicagoan originally, where it *really* rains). When the rain is characteristically light, crowds aren't effected that much. However, I was at the park three or four months ago and it was pounding rain. Crowds were virtually non-existent and nearly everything was a walk-on. The drawback, of course, is that you're going to get wet. Hell, you'd be dryer riding Splash Mountain ;-). But if you can tolerate it for a few hours, they'll be time well spent.
As an aside, I'm always trying to second guess/estimate crowds. Rarely am I successful. For example, I went to the park on Super Bowl Sunday this year. I hoped/expected crowds would be light. They weren't. I've since given up trying to game attendance.