I just don't like how I am forced to update my OS every year or so for new things because my current operating system will no longer have support, and then in this instance update my computer every couple of years to use these new Operating Systems. I have a 10.6 machine sitting next to me with some features happily working, and others I know will not within a short amount of time.
You don't have to upgrade every year. I didn't upgrade my last mac since 2007 because Apple dropped PPC support. It didn't stop me from doing anything. I usually do update the OS every year because I enjoy having the latest version.
One one hand because it was a G4, I couldn't use the newer versions of the Adobe suite. On the other hand, switching to Lion meant I have to upgrade some of my software, like ProTools, because the version I have sopped working. So there are pros and cons to both. I might fix my old G4 and use that for ProTools.
But there are no security risks from using a 5 year old OS X because there are not really any security risks for Macs to start with.
As far as support, they can only go so far back. MS doesn't update old versions of Windows anymore. The only difference is that Apple releases new OSs more often. Windows is at 7, while OS X would really be at 17.