Why don't people use MAC Address instead of UDID on iOS5?

I've read Apple is deprecating the UDID which a lot of people are rely开发者_开发问答ing on to uniquely identify devices. However, there is an API to get the mac address which should also be unique. Why don't people use that?

UDID is not just used because of being unique. It is a tracking tool of iPhone for many purposes. As an example, many companies profile cellphone users for marketing purposes using UDID. In addition, the mac address of an iPhone can be easily retrieved by adjacent machines. But, the UDID should remain private always. This link may be helpful.