Some players, including myself find games more immersive if the visual fits the reality of the universe.
I've done medieval fence and historical re-enhancement fest for a couple of years. No one would dare to put their weapons on the back, save for a bow, quiver or shield. I can get behind it with fantasy greatsword, but for normal weapons is just absurd to be sheeted on the back. I just hope those animations are just placeholders and are going to be changed in the final version of the game.
The only thing I'm a bit "scared" about is the fact that DOS2 had the correct sheeting animations so, if they were searching for placeholders, why didn't they take those ones? Perhaps to avoid further fueling the controversy about BG3 looking too similar to DO2 (which I don't believe it does too much, but I certainly understand why some people think otherwise).