Some people, especially older ones, are deaf or partially deaf, and cannot hear a bell. I've been right behind elderly people ringing my bell several times, and it was obvious by their reaction when I slowly rode past, that they didn't hear me. I also ride in areas that have lots of frogs that make a similar tone, and when there are many croaking away, it drowns out the bell.
I've found the Incredibell is plenty loud if you ring it well, even at a 1/10th of a mile away. A combination of bell and if necessary, voice, elicits the best response.