And not all software requires massive amounts of computing power. Just look at every app made for iPhone/iPad.
No, the only real reason to upgrade when everything is running smoothly for your needs is Security updates. Apple makes it real hard to keep on top of it if you're not running their latest and greatest version of OS X.
A very valid point. Apple ends support for legacy software far sooner than Microsoft.
Windows XP is 11 years old and still gets security updates. OSX 10.1 launched at the same time - when did it stop getting updates?