My helmet saved me from some bad abrasions on my head once. There were leaves all over the street, and my front wheel washed out, and the next thing I knew my helmet was sliding on the ground. I got up, brushed the leaves off and continued on my way.
Another time I fell when my chain skipped a gear slightly while accelerating hard. I lost control and fell over. I don't remember if I hit my head (not in a bad way), but I sprained my thumb from the way the handlebars twisted, and that took months to get better.
I've used my helmet a couple times while mountain biking, but that's expected. Nothing major though. I actually have two helmets, one is dedicated to commuting since it sees more abuse (being strapped to my backpack or left in the sun). My mountain biking helmet has better coverage, too. Helmets are surprisingly difficult to break, though. Next time you need to retire one (every 5 years is recommended), try to break it and you will be surprised! I was not able to break one with a 1lb hammer, and I ended up crushing it in a vise. Another time I got to throw a few in a commercial trash compactor, but that was disappointingly uneventful.