I was riding behind a guy that was a lot faster than me over in Okinawa on a rainy night in rainy season. We were rocketing (takes practice but if you are young and clueless you can master it) along with our umbrellas infront of us. I lost sight of him in the darkness for a minute and then I see this umbrella go rolling down the street past me (didn't realize it was his until a moment later). Moving on I find my friend embracing the trunk of a car with his bike on the ground (good thing he had steel rims). Anyway, it was really funny (luckily nothing but his pride was hurt). So, personally I think Umbrellas on bicycles is a bad idea having learned from humorous experience (well humorous from my point of view anyway).