You just contradicted yourself. I don't think it's pointless. Changing your IP only an expecting to be anonymous is pointless. Seeing the same MAC appear would pretty much give away who is doing what no matter what proxy you use.
I don't think what I wrote was a contradiction, I thought I had a clear dichotomy between "local area network" and "internet traffic". If you are using a proxy such as "tor" changing your mac address is quite pointless, any traffic that goes past your default gateway the mac address is actually replaced with the default gateway's mac.
Even on a school network for example, if the proxy server is on a different vlan than the lab computers, the proxy will not know the computer's mac address you are at. Obviously it will have highler levels of application data that it will track such as the AD user account.
The point I was making is there are few situations where I could see changing the mac address would be beneficial, such as my example I mentioned before (clean access). Networking applications are much smarter than they use to be.