But then any random passerby could harvest phone numbers easily. And because of the nature of a) cell phones and b) wireless networks, there isn't really a good correlation from wifi-availability to geographic location to actual phone location. You might know all of your neighbours' phone numbers, but not which goes with whom, nor which ones are on your floor, or even in your building. And just imagine someone phoning them all to try to find out who's playing music at night.