Good point, but I keep laptops until they die. Never have sold one. Do used Macs really keep that much of their value? I can't imagine spending that much on any 3 year old box (laptop or desktop).
2003 aluminum PowerBook: paid $2800, sold after 5 years for $1000
2007 iMac: paid $1300, sold after 1 year for $1300. (It was the previous gen when I bought it, so that worked out really well.)