since it takes time to test a new version and roll it out, you need to have the support for the old version extend at least a couple months past the release of the new version.
So I stand by my statement that 12 months of support with 6 month releases means that you have to upgrade every release to remain supported.
Ubuntu LTS with 2year release cycle and 5 years of support allows you to skip releases there as well. But I would actually argue against doing so, there are enough new features over two years, and enough bugfixes (most of which are not going to be backported, to Ubuntu or RHEL) that after two years or so you really should upgrade.
RHEL being supported for 7-10 years is a bug, not a feature as far as I'm concerned :-)
It allows people to stay so far behind that when the support finally does run out they have probably had close to complete turnover of their staff (very possibly a couple of times) and so end up without anyone around who knows how the apps/systems were actually built. This puts them in a real bind when they are forced to upgrade.
Besides, I just don't believe that any company has the resources to properly backport all the bugfixes over that much time. Linux development is just too rapid.