Thing is, they don't just send a gorilla to repo the car and trolley it back to the garage asap. Usually, the guy comes down with a loan car, does a walk round with the customer of the car he's leaving, and also with the car he's collecting. Both people have the opportunity to check over both cars and make a note of the condition. At that point the customer says to the bloke "I've checked the mileage, and I've made a note of the fuel in the tank. I don't expect any speeding tickets through the post while it's on its way back to you or the return journey."
Bloke then gets in the car and drives it away to be repaired, and then when it returns they both do an appraisal of the two cars again. To be honest, I'm not particularly happy letting anyone else loose behind the wheel of my motor, but if it has to go to be repaired at someone elses expense I'm fully aware that at some point they might have to drive the bloody thing.