***Steps up on soap box***
See this article for an example of why I insist on securing networks when folks ask for my help or opinion.
The summary is that an elderly couple was recently threatened to pay over $800 to avoid being sued for illegally distributing a racing video game. The threat was based solely on evidence that their network IP address was involved in the file share. In this case, the charges were dropped when enough bad publicity occurred since the couple was obviously not involved. It was not reported exactly how their network IP address was used in the alledged illegal activity.
One way this problem could occur is if the couple had an unsecured wireless router and a nearby irresponsible or malicious individual decided to switch to their network. That individual could be a neighbor or could be someone who parked out at the curb with a laptop for a few minutes. If its your network, whatever that person does becomes your problem because it points back to the IP address your Internet Service Provider (ISP) assigned to you. ISPs are required to log which customer had which IP address at what times and they willingly provide that information when requested by litigious companies. ISPs generally do not risk getting sued themselves to protect their customers. Lock your wireless network down with Wireless Protected Access (WPA) and a strong password, and you can't be exploited this way.
***Steps down off soap box***