Generally, people buy supercars as a statement, of money, power, and influence.
There are the select few who do seriously drive their supercars, Roland Atkinson (if I misspelled that, sorry) and his McLaren F1 being the most-known. If I had a supercar, I sure as hell would be driving it.
On cleanliness though... I don't like drawing attention to myself, and the people with the perfect uber-polished shined-out supercars... while I think it's good as a general statement for people to WASH their cars (especially the drivers here in PA), I don't like that 'high-class' stereotype that seems to go right along with it.
I do agree with carracer here, though, keep the thing clean if you've spent that much on it already.