Given that it costs you nothing to upgrade to the latest OS X now, why are people still running the old version?
Because they can't, or don't want to. I have some applications I've used for many years, that don't run without Rosetta. Since versions of the OS later than Snow Leopard have no compelling features I want, and some UI changes I actively dislike (open folder in new window, anyone?) there's no reason to update. plus, of course, the fringe machines that CAN'T be updated, but are still completely usable.
I have lots of Macs, and some are on the current OS - but I prefer to use the ones that aren't. For me, 10.6.8 is the Gold Standard.