You can have a look at Regarding Mint 9 LTS Isadora
to understand LTS philosophy.
Regarding Maya and Nadia, all features of Cinnamon and MATE can be installed by enabling backports.
Mint 14 Nadia has better software manager and Cinnamon is fast evolving.
If you are using it for lighter works like listening music, surfing net, then older version of software will work for you. So upgrade only if you need it. Rest depends upon personal choice to update or not.
Some do not upgrade regularly using upgrade manager but simply install new version after 6 months. I would do that only if my pc has no important data and that data will be stored in separate partition. It's not recommended, as security updates are recommended, but if you are not doing online shopping or money transfer, then you might think of it - at your risk. In general system is stable. I have many distros. To one distro I did not update it, as I never logged in for many months, but system was working fine tough the software were older.
I keep atleast 2 linux distros, one for backup + XP