It's a terrible way to secure a network. MAC addresses are easily spoofed, and without encryption anyone can sniff your traffic anyway. Even using WEP is better since then there's (usually) a requirement to see a connected client for longer than a few seconds in order to break the encryption. The only reasonable approach for a home network imo in practice is WPA2 PSK with a decent password.

Convenience. Easier to give a visitor a password than get the device's MAC address and enter it into the router's whitelist.