Depends on the situation and with whom you are riding. We ride our tandem with mostly with experienced riders, and you betcha we holler "on your left" when we're passing with a 10+ mph speed differential and they're out in the middle of the lane like no one could ever pass them. On bike paths and similar situations, we start hollering way back, but more words and repeating, like "we are coming on your left". Just a "on your left" doesn't give inexperienced riders enough time to figure out what's happening, hence the variations in reactions described in this thread. All the same, without a warning, peds and slow moving cyclists are just as likely to move unavoidably into your path as they are to keep going straight ahead. Hence the need for a warning. Those who don't warn just haven't had an accident because of that yet.