Hi Dana, I'll give you a real life, albeit embarrassing, reason for changing a MAC address.
When I was in high school, a student made a lot of fun of my body. Said I was a "twiggy" (those old enough out there will know what that means).
Well, recently I was back home, and I saw her, and she looked positively huge. She must have doubled in weight. I wanted to get her back. So, I logged into NetZero from a blocked phone line, and I sent her a message calling her all sorts of names to get her back. I even said I was a "friend" of hers way back when but I thought she looked like a pig now.
Point is, I figured the only way they could track that email I sent her was through my MAC address since the IP address would have been registered to NetZero and the phone number I called from would have been blocked.
Isn't that a case where the MAC address change afforded me some privacy? Or did I give myself away even then?