Neighborhood watch signs aren't quite perfected for the benefit of the thief. They're mass-produced and if you're on foot, you can read them from a short distance of about 5-6 feet. The graphic is a bit indeterminable at any distance outside of that, too. I'm not sure how a burglar is going to take the time to slow down - pause, even - to snag all that copy. He or she surely has other things going on that requires greater attention.
These signs may make your neighbors feel more secure, knowing that a warning is posted stating someone will call the local law. Would this make you feel safe?