For many consumers price still plays a huge factor, and using plastic will allow Apple to reach more consumers without lowering margins. If Apple does not want a repeat of the Windows era, it will cater to all consumers.
The Mac is only around today and profitable because of new product categories that gave it an extended lifeline. The curve and profits of a new product category are greater but decline as competition catches up. The competition caught up and overtook the Mac in sales making it's ecosystem slowly become irrelevant, and the same thing is happening now with Android. Hopefully Apple has learned it's lesson. Thankfully with the new iPhone 5c, it sure looks like they have. It looks like Cook is on the ball and knows the time of day.