GM Edit: Consider this a house rule.
Originally Posted by Genocide Device
After PD was removed from GURPS the inherent defense bonus from a shield became almost useless. Shields now only provide a single extra active defense which you would get from an off hand weapon. Moreover an off hand weapon skill can be improved to add an addition attack as well... so why have a shield which now effectively ties up the use of you off hand for no reason? This does not properly represent the inherent advantages to having a "MORE" defensive skill when there is no added defense.
To represent the inherent bonus to a more defensive skill I allow all players with shields to use their shield instead of their dodge so long as the shield is readied. A player gains two active defenses to "Ready their shield." this means moving to address a flanking opponent or setting their shield after moving. an extra ready is gained at level 16 and 20.
Maneuver: Shield Bash
An attack with your shield to do trow/stun/push damage. Damage thrust bashing -1. Shield must be readied once a bash has been made.
I disagree, the Shield skill is invaluable as it is.
Dodge is one of the hardest defense mode to increase, and while it's also the most powerful (basically, if you have Dodge at 15 for all intent and purposes it is impossible to hit you unless the enemy has a rapid fire weapon and an extremely good skill level with it).
Parry has some benefits over Shield but requires massive specialization and strategy to be effective.
Most weapons with decent damage are 0U (can't defend and block in the same round), and every conservative parry with any weapon except fencing weapons are at -4, which in reality limits most parries to a single one per turn, or two if you are extremely proficient with the weapon (skill level of 20 gives you defense at 13 and 9).
Fencing weapons can't block many weapons (Flails for instance), and are extremely frail and light, resulting in the fact that they either can't defend against large weapons, or are prone to breaking if the rules are followed throughly.
Shields on the other hand are the easiest skill to increase being DX/E, are capable of defending against ranged weapons and provide bonus to other active defense.
Beyond that fighting effectively with two weapons requires alot of specialization, which is not always viable for most character types.
As such the shield skill is very useful, for instance in another game I have a character with a parry defense of 15 with weapon master (that effectively reduces consequentive penalties to -2) still has a shield skill.
The only modification I see viable is that the blocking isn't limited to once per turn, and should be given the same penalties as parrying.