I agree with the first statement, but our experience has been there are those who are looking for a free place to stay and don't want to learn or contribute. Those are the ones we seek to screen out. It's happened a few times to us, always when the references weren't great.
Troubled pasts don't bother us so much as not having good references for who you are now.
Yeah, I understand you don't want some lazy "zombies" lurking around on a farm... That must be horrible... =)