Filtering by MAC although effective against the casual Wi-Fi sucker is not effective against someone who is determined to get into your network. Besides, the WPA/WEP passwords are not just to keep people out of your networks, but also to encrypt the data you are transferring to and from your router.
I would highly discourage anyone from just using MAC filtering as a line of defense against their network, it is barely a fence. If in doubt, read this tutorial that shows you how to bypass it..
Wireless: Bypassing Mac Filtering