I would definitely keep some type of security on your wireless network; not only does it keep people from stealing your wi-fi, it helps keep them from poking around inside your network if you have any type of file sharing enabled. I had my wi-fi open at one point (to accommodate on of my kids gaming systems), until I got a call from my provider telling me that I had over 750GB of activity the previous month. I assume one of my neighbors was running some type of bit torrent over my network.

None of the security provisions are perfect, but they are usually good enough to discourage the casual thief.