I was in a heavily crowded region that was lagging me to death so I clicked a random LM in my inventory, which just happened to be on someone's parcel but I also had to go AFK right at that moment. When I got back I'd understandably been kicked from the parcel. Only she hadn't ejected me but actually pushed my avatar right out of the parcel either with her avatar or a weapon and had been stood there hurling abuse at me for 5 minutes straight while I was away, insulting my character and my RL profile picture. So then her partner and the region owner comes in, and I get banned from the entire sim for "trespassing and being abusive".
So if you don't actually WANT people in your land, why keep it open? That's why the option is there. Close it off if you don't like visitors. It's just the logical thing to do. If I come to your land and it's open with no security I'm just gonna naturally assume you don't mind people on it. Visitors on your land that are doing nothing harmful doesn't give you the right to act like an abusive psycho towards them. But as ever people are still messed up in this game and take it way too serious.