I thought the same, until you try it, yes it hardly looks convincing but it works 100%
People actually walk into you sometimes and bump off.
after you kill the same guy once or twice they wise upto it, you have to move.
Think about it when you walk into a room first thing you look for is guns pointing at you, then Claymores C4, last thing you worry about is a motionless guy on the floor without any weapon equipped. The fast weapon switching kinda suspends the animation in a zone where you are mid swap. I even did it with an AT4 on my back.
try it - im sure they will fix it but its funny as hell - obviously you need to be playing with some proper idiots too to help :O