I suppose it depends on whether you're worried about someone thrashing the bejesus out of your bike and having it cartwheel down the track shedding parts as it goes, or some sort of mechanical failure.
Realistically, a mechanical failure should be the bike owners problem if the the bike was just being ridden as per usual.
I'm imagining most guys would have a bit of respect for the fact that they were riding someone else's bike, and do their best not to throw it away.
I'm pretty keen to have other people ride my bikes to get another opinion on setup, as I don't have a clue what I'm doing