Also, in my personal opinion... the vast majority of the centered versions of the weapons were pretty ugly and/or made absolutely no sense. Most notably those of the Combat Shotgun, Heavy Assault Rifle, and Rocket Launcher.
Not to mention, you don't get to see as much of the beauty of the complexity and intricacy of the weapons so much.
And also, holding them angled is much more natural than holding them centered, and even looks the part... unless you're taking a careful and precise shot. But even then, it's difficult to naturally be able to hold the weapon truly centered.