I do agree with you on this in theory but to be honest I personally hate this. 9 times out of 10 the person shouting "on your left" startles me and my brain goes into overdrive frantically trying to calculate which side he is on with respect to my own position and trying to get my limbs moving in the opposite direction. The twist is that I'm usually not in the way to begin with most of the time.

Now I'm not sure how to solve this conundrum but I'd rather be more inclined to believe we need better design than resorting to strangers shouting their ever-changing location.