If I had my bow mounted like that on my pack it would get caught on something, rip my pack and rip the bow out of there and smash my sight.
Its just my luck. I want my bow in my hand at all times. If it isn't in my hand it is on the ground with the string up leaning against my leg.
I could never use that in the places I hunt elk.
If you hunt in the open then I could see its practicality.