I have some systems that I update more rarely than the others. Of course I highly recommend keeping up to date, especially for the security fixes. I've also written similar update reports in the past. You may want to read them and compare the experience:
Another 5-month update (December 2011)
Another report from rarely updated system (September 2012)
Another 5-month update (December 2011)
Another report from rarely updated system (September 2012)