I always clean the cars I get as loaners when mine is at the stealers as well......Just think that it doesn't take long, I enjoy it, I get to drive a clean car and it makes them realise that I will look after things that I borrow from them which can pay dividends ( I have been given loaners that I wouldn't otherwise have had because of it ).
It also helps come resale time, as they know that if I look after the loaner, then I will REALLY look after my own.