I have been using Ubuntu 14.04 as my main OS for about two years now. I wonder what are the advantages of the new LTS version (i.e. 16.04) so I can decide whether I should upgrade or not.
There are basically three things to consider.
Ubuntu does have a limited shelf life. Every two years, Ubuntu puts out an LTS (Long Term Support) version. Starting with 12.04, the long-term support lasts (security updates) lasts 5 years. While you can keep running any given version of Ubuntu indefinitely, it is unwise to run a computer without security updates, particularly if you are online.
Therefore, if it were just the security updates, it would seem logical only to update every 4 years or so, and skip one LTS version out of two. However, on forums etc., you will find that the current LTS version will be the easiest to find debugging, etc. Frequently, I have updated a computer to the newest LTS simply for that reason. And often, the bug is fixed in the newest release.
Though this is very subjective, there is no doubt that 16.04 has additional features. It is somewhat controversial as to whether those features are desirable or not, but in general, for the average user, it makes one's workstation more useful to be using the latest GUI (Graphical User Interface).
Many packages are more up-to-date in 16.04, meaning more functionality, more usability, and more compatibility. Furthermore, the fact that it is more up to date means that it there may be more programs available to do the job you need to do.
Basically, you're better off if you upgrade, but if it's a lot of trouble to do so, you can afford to leave it for a couple years or so.