The reason to have the sword maintain concentration is not for defense. It is so that the wielder can attack without having to make the Concentration 15+ roll and have the defense go down. The wielder must always be aware of an attack to parry it.
As for the Herbam addition, the description of Defender Swords specifically says "An active Defender Sword allows the wielder to automatically “parry” incoming blows from metal weapons."
The below is from my Input on Grand Tribunal Competition thread.
Effortless Defender
Re(In)Te(He) 30 (Base 2, T/C/I, Metal 2, Requirement 1, Item Maintains Concentration +5, Unlimited Uses +10)
When active this allows the wielder to automatically “parry” incoming blows from weapons covered by Terram or Herbam. The wielder must be generally aware of the incoming attack, so he can still be stabbed in the back by an unknown adversary. However, in normal melee he can automatically “parry” any number of blows in a round. He may also "parry" single missile attacks (arrow cutting), though not a volley. All "parries" require a roll, but only to see if there is a botch.
This is an upgraded version of Defender Swords, TtA p.94.
EDIT: If you are wondering about the changes, the Herbam Requisite is +0. The Intelligo Requisite is +1 and causes changes such as converting things such as "must be aware of the incoming attack" & "he can still be stabbed in the back" in the original to "must be generally aware of the incoming attack" & "he can still be stabbed in the back by an unknown adversary" in the improved version. The Intelligo also allows the arrow cutting in the improved version.