I always use it and friends who work for consulates around the world certainly encourage it because If something goes wrong, terriorism, earthquake, tsunami etc, DFAT will immediately SMS those who are on the system and in the area. Much quicker than friends and family back home, and certainly the most up to date and relevant info.
Say there was a terriorist attack, they could SMS you advising where to stay put, or if there was a Tsunami coming, could advise you to get to higher ground etc.
I've found doing it on the iPhone app the easiest now - much easier than the website as it retains a lot of your details and doesn't require as much info.
It's one of those things that you probably don't feel like you need it most of the time, but if you do need it, you are so thankful that you had registered.