I had the same reaction at first ("useless") .
Then I remembered my father whose hearing has gotten pretty bad over the years.
When my parents were out together riding their bicycles somewhere, my mom noticed that he clearly didn't hear some cars approaching from behind. She said that it was kinda worrying in some situations.
I guess when your hearing gets gradually worse, you can sometimes forget that not hearing a car doesn't mean there isn't one close by.
And thinking a bit more about it, I've already seen ("heard" would be wrong) an incredibly silent hybrid car; the loudest noise it generated was the dirt being crushed between the tires and the asphalt.
Then again, turning your head now and then or some of those rear-view mirrors for the handlebars will be a lot cheaper.