I have a feeling Apple knows very well how much people use certain UI behaviours from their analytics data (from those who send).
I for instance never use this particular feature/function. It isn't very obvious or quick or reliable compared to double-pressing the home button for me.
I'd probably get more use out of it if it worked on the right side, since that's where my thumb is normally resting. I have no digit normally resting anywhere near the left side. On a Plus-sized phone especially the left side is almost impossible to reach if you're right handed (like me) and hold and use the phone with your right hand.
Chalk this one up as one that benefits the lefties for a change. Perhaps that's why it's been (if it has) under-utilised.