I was once at a horse show when it rained very heavily before the ridden classes began. All of the flatwork classes were still on, however, they called all of the coaches and their jumpers together to see if the jumping class were still on. At that time, I was taking lessons with an eventing coach, so that meant as long as it wasn't thundering with lightning, we were riding. My vote was yes, but everyone else was a no. I didn't understand, as it was expected that my horse went in all weather conditions, but then again, if it rained before I had lessons at a hunter-jumper barn, we either moved inside or it was cancelled.'
Those who are willing to practice in the rain, imo, are the better riders. They don't take days off, and continue to work to improve. Get out there, as long as it isn't storming, and practice. Don't do sharp turns and excessive speed, use your judgement.
And let's not forget how horses generally act when they aren't ever ridden in sloppy arenas or in rain for that matter...they under-perform greatly when asked to do so. When my horse gave an indication of being crabby and unwillingly to move forward when it was sprinkling, I bet you can get who got ridden every single day it rained for the next month!
Toofine - 1998 Half Arabian
Minnie - 2013 Morgan