I fall into a Macbook and PowerPC category so I'll try to answer this the best I can.
Everybody I know that doesn't game or do anything remotely processor heavy like audio or video tends to keep their machines for a VERY long time. I know people that are still using generic Dell P4 towers. These are the people that don't need much, know they don't need much, and are more likely to just buy an older Mac and be happy with it.
Although my needs have grown exponentially since I got my Macbook in early 08 I can understand why theres still a market for them. Lets face it, they are pretty attractive especially my the black ones like mine.