Wi-Fi routers can give you enough headaches without worrying if someone is leeching off your connection. You pay for that bandwidth you receive every month, and with so many movies to stream, games to play, and work to remotely connect to, you don't want to share your Internet pipe.
Not only can having others access your router slow down your connection, but there is the possibility they might get you into some legal entanglements. An incident made news least year in which a man was falsely arrested in child pornography charges. The guilty party was actually a neighboring bandwidth parasite neighbor using the innocent man's connection to download the odious content: http://www.geek.com/articles/news/m...-password-protect-wifi-the-hard-way-20110426/