I think this would make a lot of people uncomfortable. I'm relatively pragmatic and untrusting, but I would be offended if someone I knew did this to me. And if it were a neighbour I didn't really know, I would be a bit worried: now I have the responsibility of keeping their key secure, and if I lose it they will suspect me of stealing it, and all for the privilege of being asked to do a favour for a borderline stranger who doesn't even consider me trustworthy.