I have multiple WiFi access points on my network. One because I don't want the speed to drop on any when a slower device connects (my 802.11 N network only allows N connections for example, so it doesn't drop to 11 Mbps or something when an older "B" device tries to connect).
My 802.11 B device is configured with WEP authentication as well. I have older (1998) laptop that uses 802.11 B, and my Nintendo DS can only use WEP, so it makes sense for me to keep up an access point for devices like those.
Of course, with WEP authentication, it makes it a target for easy hacking. To try and prevent someone from stealing all my bandwidth, I use QoS to keep the 802.11 B connection speed limited to just 128 kbps.
My bandwidth was safe, but people would still be on my network if they connected.
A solution would be to put the 802.11 B device on a separate VLAN to prevent access to your network while still allowing Internet access.
Here is one configuration using a WRT54G router, the excellent Tomato firmware, and a spare WiFi access device (I'm using a FON router flashed with DD-WRT).