In addition to all of these reasons, MAC filtering is also much more of a pain in the butt to upkeep than instituting something like WPA2-PSK. For encrypted wireless, the MAC address is either a part of the initial handshake (used to derive the session key), and/or exposed during pre-encryption communications. In order for wireless to work, MAC addresses are exchanged in plaintext (Regardless of whether you're using WEP, WPA, WPA2, or an OPEN AP).
The reason why it's not a part of the 802.11 spec is because it provides no true security (via kerckhoff's principle).
MAC filtering is not a part of the 802.11 spec, and is instead shoved into wireless routers by (most) vendors.
