I have before. I had crashed earlier in the day while it was dry and stuck around and kept riding after it started raining as I thought I've already damaged my bike so what does it matter if I go down again. I was on my XJR1300 at the time and I was enjoying the wet with having the back end sliding around a bit. Obviously I wasn't as fast as in the dry, but I reckon I had more fun in the wet afternoon sessions than I did in the morning dry sessions.