If it works - why break it?
Because upgrades rarely break anything, fix problems you may encounter even if you haven't yet, and fix security issues. The bulk of the "I upgraded and it broke" reports here and via support customers end up being not upgrade related at all, or things that weren't actually right to begin with and only worked coincidentally.
Though with every major version change, there are possible behavior changes and other things to watch out for, which we detail in the upgrade guide.
You forgot a major aspect of the upgrade reason – new features!!
Here is another reason to upgrade.. Other than the huge part of about security patches, if you are running an older version and come here for support people will just tell you to upgrade before you get any help.. This is the same with every major software/hardware company..
As the security device, I would really suggest you keep it updated if nothing more than for the security fixes..