not for authenticity, just cause it looks better than a weapon arced across the back and jutting above the shoulder.
i liked the way the last of us attempted to correct this, and by extension the way tomb raider did it in '96 (shotgun next to, fastened to backpack), but it kinda breaks down with larger weapons. in that case i guess the solution is easier - no giant weapons sprawled across a character's person - but even then i think slings work better. your weapon hangs by your hip and looks cool while never really getting in the way.
you might think a sling means extra animation means possible physics simulation means extra work for designers and animators, but you can easily cheat it too, and without losing the look or feel. take ground zeroes/the phantom pain: i don't think big boss even has a sling. his weapon's just hanging there at his hip, about as attached as any back slung weapon in gears, lightly jostling with his movements. whether that'll change for the final release is anyone's guess, but it doesn't really have to because it looks good without much in the way of extra work or processing. and you can solve the problem of believability if you really need to by going with a modeled but tightly worn sling that only sort of rustles with movement. doesn't need any costly cloth simulation to look right.
i realize it's a personal thing, bordering on obsessive, but that the standard as it is now remains pretty awkward and i think many would agree. messes with a character's silhouette. i'd like to see that change. (always preferred sword and sheath at the hip, too)
edit: i guess a modeled sling would entail some extra animation unless you just want players to imagine your protagonist is clipping/unclipping his weapon each time, which is kinda how i imagine mgs5. but even then it doesn't require a bunch of fancy rigging or simulation if you're doing some three-point sling kinda thing. mgs2 even did as much in 2001 for its enemy soldiers, with minimal simulation.