I always ride with a cap, rain or shine. I rode in a light rain yesterday and the cap definitely reduces the amount of rain on my lenses. I use the Voler Caliber cap, perfect material (thin) and the best bill I've found (big enough to make a difference but doesn't hinder line of sight). This cap also is easy to reposition while riding if I need more eye coverage.

I ride mostly solo (especially if roads are wet) so spray from a rider in front isn't an issue.