This study proves what I've been suspecting for years. Flushing toilets contaminates the room with a plume of pathogenic bacteria! I always wonder what wild animals I must live with when I enter a bathroom with an opened toilet lid. Even if you're lazy and a man, keeping the lid down is an easy maneuver, takes milliseconds, and clearly provides you with tons of protection against the many infectious things in toilets (poop, puke, pee)!
Outside of the obvious safety issues - protecting your pets or small children from falling head first into a slippery shallow puddle that they may not be able to escape from - keeping the lid down prevents the toilet contents from being thrust into the air and coating everything else you keep in the bathroom (toothbrushes, combs, razors) - these are things you want to keep germ-free, right? (BTW, I keep my toothbrush inside the medicine cabinet for extra germ protection - you should, too!)