Rain of toads causes small army disease--not nearly as much as Leprosy, but it has the added bonus of affecting the province it is cast at. It will increase the misfortune scale (similar to Baleful Star) which can be very effective if your opponent has gone for Misfortune scale to get extra design points. It will also increase unrest in the province thus decreasing income. Leprosy targets only the army but does so very effectively. I think RoT works kind of like Fires from Afar (the little one), affecting some small random number of troops. It's still a useful spell, you may disease a few units plus you injure your opponents economy and cause unfortunate events like tax loss, lab fires, earthquakes, knight/hero attacks or even worse events like unruly jailers
