It's a great service, never been bombarded by emails and it probably saved my life. Was in Beijing couple of years back year during a wave of severe smog over the city. They sent out an email strongly warning of this and advising Americans to stay indoors, I followed the advice and even indoors in my modern hotel with functioning central AC I was still struggling to breath. I have asthma which is typically under control with medications, but even my rescue inhaler had very little effect. I had been to Beijing a few times in the past with moderate smog and it didn't bother my asthma, but on this day the smog was in another league. Without that warning I certainly would have gone out sightseeing, no doubt bad things would have happened to me without that warning.
I know of another instance of an American who was volunteer teaching in a remote part of Lebanon when the Israel-Hezbollah war broke out in 2006. Because she registered her stay, the US embassy was able to find her, and transport her to an evacuation ship to safety to Cyprus.
In yet another episode I was in Mexico City with a thousand other US soccer fans when the national team was playing a World Cup qualifying match. After the match a bunch of smart-ass US fans decided to hang a large American flag outside the window of the hotel the traveling fans were staying in, which happened to be across from a monument where Mexican fans congregate to celebrate a victory. The locals didn't take the flag well and hundreds of fans attacked the hotel and riot police and to surround the hotel and literally beat back the mob. The US embassy was adjacent to the hotel, they placed a spotter on the roof, and texted minute by minute details of exactly where the mob was and how to navigate around the police line for Americans stuck out in the streets.
While the above scenarios are unlikely in Western Europe, natural disasters can happen anywhere.
So in my experience the STEP system has worked very well.