Wild ones are plague carriers and can be very destructive to buildings and automobiles. We had a family of rats living under our shed at our last house. I decided to just let them be, and they multiplied like crazy. They turned my shed into a toxic waste zone, and got so numerous that they were running past us, like right in front of my feet, in the middle of the day. That's when I decided that they had to go. I didn't want to hurt them, but they spread like mad, and can't really just be left to be unless they're in a giant field or something. One rat can produce 144 baby rats over the course of a single year. If you count the offspring of the original single rat's offspring, they can produce up to 100,000 rats in one year!