The theory is fine, but its implementation is flawed. The first time I used it, water sprayed onto my screen and the wipers didn't come on. What possible reason would cause me to want the spray and not the wipe?
Given that if someone is spraying their screen (presumably to enhance visibility) then the single action of spraying the screen should then activate the windows (even after 2 seconds or nice and wet) - but you should not have to hold the stalk down.
In any case, its just the way the 500 works. Personally I would prefer the two actions to be combined, or a single tug on the stalk to do both.