If you don't think negative externalities matter, here's a nice example from my morning today.
My next-door neighbor does an excellent job taking care of her home. In fact, last evening she had been doing some weeding and edging just before dusk fell. Because darkness came sooner than she had expected, and because she left early this morning for work, she did not have a chance to clean up after. As a result, she had left the weeds and cuttings from the edging strewn about her front walk.
Well, my house in Holland, Michigan, is currently for sale, and our agent had arranged for a potential buyer to come by for a showing today around 11 a.m. So around 10 o'clock this morning I was next door, carefully sweeping up all of my neighbor's weeds and cuttings so that my block would look a bit more presentable when the possible buyer pulled up to take a look at my house. Don't want anyone to get the mistaken impression that we have a bad block!
I hope my neighbor enjoys the nice thing I did for her this morning. At the same time, though, I sure hope my other neighbors don't read this and start brainstorming projects they could strategically begin and then leave unfinished, thinking I will stop by to tidy up the negative spillover they've left.