Well, think about it in terms of strategy-based games. You can only attack once every x seconds, regardless if you do 9.9 damage, it will still take you two hits to kill something with 10 HP.
Nano saber offers the most efficient DPS. Allowing you to one shot mobs. The next most efficient (imo) is the chainsaw. 5 damage, twice is 10. Unless the mob has more than 10 HP or it is a boss mob (which you should really be using something like a bow to kill), anything in between 5.1-9.9 doesn't offer anymore returns than a weapon at 5 damage. For most vanilla mobs anyways. I am not sure about what's in TF or what the other mod packs add.
Just FYI, the only way I ever 1-shot witches at melee range was with dashing attacks with TiCo longswords or damage amped TiCo battleaxes. So you CAN reach that threshold and one shot the most aggravatingmob in the game.