Its been brought up many times before in the past here.
To an extent, hacks are good but if they become to overused they become annoying and it won't be fun anymore.
It gets to a point that in a game there's going to be hack vs hack situations which suck. You can't even play the game if that happens.
The purpose of the head hit box is to clear out CO-OP matches. There's no point to use that in PVP because it kills the purpose of the game.