There are some security holes that have been patched which should be reason enough for an upgrade!
The changes between 3.1 and 3.1.3 are mainly bug fixes and theres a hell of a lot them! If theres a bug that annoys you in 3.1 the chances are it'll have been fixed.
And finally to help improve KDE it helps if you are using the latest version so that you can file relevant bug reports.