ORIGINAL: BuzzBomber
I run into this problem more often with planes than with cars, and I will not let ANYBODY fly my planes unless I know them and I am familiar with their flying. I did let a stranger have a go with a lightweight electric trainer once, after he convinced me he knew what he was doing--he actually did fine, but I resolved not to do it again anyway, after thinking about what might have happened if he were a better storyteller than pilot. With cars, I've only been asked for a turn driving once or twice(probably because I try to stay AWAY from other people when I got out bashing). I did let a friend's 6 year old drive my MGT with the throttle endpoint turned down all the way - he actually did very well with it and now has his own Ultra Comp LX(at 6 years old!). I guess it all comes down to whether you can trust the person asking for a go at the wheel.
I don't blame anyone for not letting strangers fly their planes. Especially any nitro powered plane. They are a different animal. One slip and several hundred dollars and many, many hours of work can be completely and utterly destroyed.
Cars/trucks on the other hand, What's really gonna break? Unless somebody drives it under the wheels of a moving 1:1 car it's unlikely anything expensive is gonna get broke. Certainly nothing worse than I'm likely to break myself. I will let anyone who asks politely drive any of them that I own. As I said above, letting others find out how much fun this hobby is, is IMHO one of the best things about it. I'm really finding it an eye opening experence seeing how many people take the "it's mine and you cann't play with it" attitude. I must say I'm really, really surprised!