Tunnie is right, Xenon equipped vehicles need headlight washers, every single xenon equipped vehicle sold in the uk has headlight washers, without exception and this is mandatory.
Wipers were only fitted to V70's and S60's until 2004 '54' plate and the L322 Range Rover from 2002 until the facelift in 2005. They then just changed to headlight washers which are more effective then wipers. I can't think of any other newish vehicles with headlight wipers fitted.
Having driven both halogen and xenon equipped vehicles, the light output is greatly increased when the headlight washers are used on xenon equipped vehicles. This is the case on Omega's, but even more so on some other vehicles, including the L322 Range Rover.
mmm mostly correct. Wipers essentially got removed due to legislation about non deformable structure on the front of a car
HIDS run cooler, but the dirt film increases their dazzle effect, hence needing washers
and they are only required on vehicles with reference lumens of 2000 or more, so not essentially mandatory on all vehicles, although to be fair all manufacturers with HIDS/Xenons do tend to fit them
Simon is bang on the money!
Due to the intensity of the directional light through clear lenses, any dust or dirt particles on the lens will refract the light in various directions, potentially causing dazzle to oncoming traffic or vehicles in front (which dont have the auto-dimming rear view mirror

).
For this reason, the powerful washer jets are a legal requirement for vehicles fitted with this type of forward facing lighting.
Interesting how 'Mikes1670' stated he removed them because they were 'ugly'. For anyone who knows about cars, they certainly prove you own a vehicle with the option boxes ticked.
I can only agree with others, that when the washers are used, the visibility increases immensely. I am annoyed that they use SO much washer fluid. And, when the washers are in action, the forward lighting drops by a significant amount (if only for a second or two).