I let pretty much anybody at my local ride my scooter for a bit if I'm just sitting down not riding it. If I'm in the middle of trying to land a new trick, no. If someone else just rode it, no. The reason is, I know they aren't going to be able to mess up my scooter short of flat spotting the wheels. I remember when I just got my first Ultra Pro, the first time I let someone ride it he flat spotted the back wheel by holding the brake. Pissed me off so much I didn't let anyone ride it for a while after that. But I got over it. Most people just take it, try a 180 or a tailwhip, then give it back and say "Fuck that, I'd kill myself on this thing" and go back to skating haha.
EDIT: It all depends on how comfortable you are at that park, how many people you know there, and the area where you live. If I was riding in NYC, no way in hell would anybody be allowed to borrow my scooter except close friends.