Joe I agree with you 100% - Corgi's points are sensible and well argued but I would say the reason the washers are there is to comply with regulations! They do not provide any significant cleaning action in day to day use. I guess if you went through a rally stage to the extent the lights were covered in mud and completely blocked they might have some use but day to day, no.
Exactly! For a layer of wet mud, then I can imagine that they'd be some use.. but for day to day road crap (including baked on dead flies, etc), they are useless.
Also, they do make the lamps dim for a couple of seconds when in operation, which is a little scary!