I let my brother drive my tractor, sometimes. He operates a skidsteer almost daily, so he's a little harder on it than I am. Other than that, I agree with everyone else, if they want/need the help they get me and the tractor, too much at stake.
Reminds me of a motorcycle ride with a buddy a few years ago. We were riding in a pit behind my house, and he rolled one of my bikes. After the dust settled, and I confirmed that he was ok I looked at the bent handlebars, and the 6 inch long crack in the rear fender. His respose as I restarted the engine was "at least nothing is broken". That was the last time he was invited over to go for a ride on my machine.
I work with a lot of different tools daily. I don't lend out anything that I use to make a living. However, my family and close friends have often won the tool lottery, and have been given nailers, compressors etc. I tend to upgrade, or by a tool that wasn't up to par. When the consistency of a tool goes away, It doesn't make sense to keep it as a pro, but my brother has a pretty decent set of my cast offs.