Ok, let me clear up some misconceptions:
Wireless Internet Access and a Wireless Network are not the same thing. Wireless Internet Access is like Wired Internet Access (e.g. cable/DSL) exception it doesn't use a cable for the last mile (between your ISP and your house) and it is slow. A Wireless Network links computers to your home network and/or internet connection. It is typically a variant of 802.11 (a, b, g, i or n; take your pick) and is fast (11mbps on b and i if I recall and 54mbps on everything else).
WLAN means wide access network. It is a type of network but not a wireless network and definately not a home network. The most notable example of a WLAN would be the internet.
Having a Wireless network without encryption doesn't mean people will be able to get files off your computer (although it increases the chance) nor does it mean they'll be able to use your internet connection (although it increases the chance). It means anyone with the technical know how will be able to easily and legally interpret transmissions between your computer and access point. I run my home network without encryption and have yet to be hacked into for a few reasons:
1. Few very people around me are technologically savvy.
2. It necessary that someone first break into the network to break into computers and I have a Mac ID (a unique identifier for a network device; it is unique to every device, even if they are the same model) filter on my network which restricts access only to wireless network cards I have cleared for access. Of course it is possible to emulate the Mac Id but it is also possible to crack encryption.
3. My computer is secure. Every account I don't use is disabled, my computer has a firewall as well as every patch from various software manufacturers installed and passes Microsoft's BLSA's tests:
https://www.microsoft.com/technet/security/tools/mbsahome.mspx
In short, anyone with a wireless network simply has to use a Mac Id filter and their network is relatively secure against unauthorized entry. I would recommend securing any and all computers on the network like I secured mine but that isn't necessary for the sake of keeping someone out of a typical wireless network. If they are either concerned about the data being transfered being captured and interpreted or really paranoid, they could turn on encryption at the expense of wireless network performance.
I hope that clears things up.