I wouldn't say it's useless, but it's confined to very special cases, like when you're going to enter a building and expect enemies in there. My main problem is, that the current system just doesn't feel authentic. Suppose two riflemen meet and miss both shoots, now they start to run around each other, cycling the bolt or attaching the bayonet, and look very stupid. Thats not what you are expecting they would do, normally they would charge and try to stab or club the other guy. It's even worse, if one of them has a smg, the rifleman runs around even wilder, trying to dodge the bullets until he gets his weapon ready to attack again.
Therefore by shortening the time to switch to "stabby mode" you get a more authentic, natural and smooth gameplay.