It always rains when you least expect it.......
definitely true, but that doesn't mean you have to pack for a monsoon. If the only rain gear you have is a disposable poncho, having a full 3 person tent seems like overkill.
I might suggest a closed cell foam pad; while they can be pretty bulky, they are light and cheap. I tend to put mine into my dry bag (strapped to the rear rack) first, and expand it so it works kind of like a frame.