Personally, I don’t see any real sense in this.
First off, I think the current system works just fine, and there’s no need to change it. In fact, a changing backstab angle depending on the weapon would bring in unneeded complexity, and the stabbing (or its damage bonus, at least) is already of questionable effectiveness without Bloodfletcher.
That’s more out of practical reasons than damage efficiency. Approaching a moving enemy from behind is safer than from the front, and easier and safer than from the side. While headshots bring in a bigger damage bonus than backstabbing (at least on the weapons most Shades use), they’re hard to aim and timing-critical, unlike backstabbing, so might as well take at least some bonus. And of course, people are most likely to run Bloodfletcher which also works on backstabs, so that gets some ammo back at the same time. So the backstab damage bonus by itself isn’t likely to entice people to approach from behind while Infiltrated.
Besides, this would bring in a curious imbalance to the weapons, and one that I think goes the wrong way: The bigger weapons, especially the Spear, would be the weapons to backstab with. The weapons with a good Cleave are already very good to slay groups (and to gain ammo with Bloodfletcher); this would increase the disparity. While Spear has the biggest block angle and would certainly gain an advantage from this (which I already find questionable, style-wise at least), the weapon to gain most would probably be Dual Swords. They’re already capable (in skilled hands) of dispatching two Elites on one strike, but getting an increased backstab angle would, combined with Murderous Prowess and their increased crit rate, make them one-hit kill a vast majority of enemies - anything considered “Man-Sized”, really.