Among several Mac laptops I have I regularly use an original MacBook 1,1 - Core Duo 2.0GHz - 2GB ram that was purchased 5/31.2006. It has remained useful even though I also have a 2010 MB Pro.
The installed OS X is 10.6.8 (Maximum) which is mainly what makes it useful. I have the advantage of Rosetta to run PPC software such as AppleWorks alongside it's replacements Pages, Numbers, and Keynote. When I upgraded the 2010 MBPro to 10.7 I lost the use of a printer and a scanner which were not supported. These have remained connected to the MacBook for use.
It also has a BootCamp partition with Windows XP for some of the older games that are not so graphics intense like Half Life and it's add ons.
What's new is not necessarily needed and what's old is not automatically obsolete.