I have a slightly different perspective than some it seems. I would never dream of thrashing or abusing something which does not belong to me.
It's not my property to do so with.
As for abusing a courtesy car, i own a couple, or at least my company does and if i found out they were being abused or mistreated i would not only withdraw that privilege but i would absolutely ensure that any damage was paid for, and then some !
Look at it like this; Everyone wants the best price for work on their cars, they may also want a courtesy car so do you think companies, especially small ones, conjure up the money for such luxuries out of thin air ? No, obviously not, it comes out of the company profits, if a percentage of those company profits are spent repairing the damage to engines and transmissions caused by half brained dim wits then ultimately the costs of labour/work etc has to increase to pay for it.
Insurance fees for motor traders are astronomical, probably made much higher by people smashing up such luxuries as courtesy cars. The more it increases, the more they have to charge people or you end up as a charity.
Think about it !